Whistling noise from the rear after 40MPH

junkwallah · 11-02-2009, 01:36 AM

Hello All,

My old Impreza is making a noise and I hope you can help me track it down.

The car is 2000 Impreza Outback, manual transmission, with 105K miles.

The noise is a whistling noise coming from the rear. It becomes audible at around 40MPH, but not at slower speed, and varies its pitch with the speed. At around 80MPH, the high pitch and faint volume is almost, but not quite, masked by the wind noise.

It seems to be emanating from the wheels, but not certain which. At times it sounds as if it's coming from one side, at another time from the other side.

Wheel bearing is a suspect, but the pitch of the noise is rather high compared to failed wheel bearings I had on the car previously. Also, the noise seemed to almost "switch on" at around 40MPH, which seems odd for failing wheel bearing. Each rear wheel bearing has been replaced less than 30k miles ago, at different times, using the updated taper-roller type.

I checked the brakes (it has drum brakes on the rear wheels) and found nothing loose. I did notice a slight binding as the wheel turn part of a circle, but I believe this was an existing condition long before the noise started. It seems to be more pronounced on one side (left-right), but can't tell for sure as turning one wheel causes the other wheel to turn in the opposite direction. Also, it was difficult to localize the cause of binding - hub, axel, differential, can't tell.

Replaced the rear differential oil for the heck of it, and found bit of fine metal scraping stuck to the drain plug's magnet. I think that's normal? The oil was last replaced 30k miles ago.

Any insight?

Thanks for reading.

Additional info:

The "switching on" of the noise at around 40 MPH isn't quite so abrupt. The noise is audible at below 40 MPH but the volume is faint and the speed-dependent pitch is low so it takes an effort to discern it. At around 40 MPH, it seems the pitch reaches the resonant frequency of the whatever is making the noise such that the noise volume increases and the noise becomes apparent. Past 50 MPH+, the noise volume decreases again as the pitch moves away from the resonnant frequency.

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Bayley · 11-02-2009, 12:33 PM

If the noise changes pitch with engine load (acceleration verse deceleration) it is likely something related to the rear ring and pinion gear. A worn ring and pinion set typically makes the loudest noise while "floating". This is when the car is neither accelerating or decelerating but at speed.

The metal shavings (which shouldn't be there) are another clue pointing to the differential. A broken or chipped tooth from the carrier gears likely could have found it's way onto the ring and pinion surface initially causing the binding and then eventually the noise. All three R160 failures I've had were a result of this exact failure.

Your best option is likely to replace the entire differential with a used one from a junked car. Good news though, the R160 differential has been in use since the late 1960s. Finding replacement units is pretty easy and comparatively inexpensive. Trying to rebuild any differential is always a more expensive option and usually only necessary when trying to find special / custom rear end ratios. I believe you have a 3.54 to 1 ratio, which is one of the most common R160 ratios available.

Good Luck!

junkwallah · 11-02-2009, 03:54 PM

Bayley,

Thanks for the reply.

I'd appreciate it if you could share bit more of your experience in regard to the mode of your failed rear differentials, so that I can plan for its possible replacement. Was it pretty abrupt failure since the start of the noise, or was it prolonged and gradual? If gradual, how long was it before you decided the diff needs replacing? Does the noise volume increase? Has it ever completely wear out on you? The noise "switching on" at around 40 MPH, was it also a characteristic of the failures you have encountered?

I wonder how much shaving in the differential drain plug is normal. I'd guess some shaving is normal since there is a magnet in the plug, but I can't recall how much I saw the first time I changed the differential oil.

Curious also about the typical lifespan of the rear differential. Although I at times have driven it bit harshly (especially with sharp turning), I have not driven it like a street racer, and the entire power train is stock 2.2 liter NA with no modification of any sort. It would be very disappointing to me if the rear differential indeed is worn out at 100k.

Thanks for sharing your insight.

junkwallah · 11-02-2009, 04:05 PM

Also, the rear differential assembly I found online, a new OEM part, is noted with 3.900 ratio, and is priced at around $750. I wonder if this is the right part, and if I can do better price-wise.

11-02-2009, 01:36 AM	#1
junkwallah Scooby Newbie Member#: 228149 Join Date: Oct 2009	Whistling noise from the rear after 40MPH Hello All, My old Impreza is making a noise and I hope you can help me track it down. The car is 2000 Impreza Outback, manual transmission, with 105K miles. The noise is a whistling noise coming from the rear. It becomes audible at around 40MPH, but not at slower speed, and varies its pitch with the speed. At around 80MPH, the high pitch and faint volume is almost, but not quite, masked by the wind noise. It seems to be emanating from the wheels, but not certain which. At times it sounds as if it's coming from one side, at another time from the other side. Wheel bearing is a suspect, but the pitch of the noise is rather high compared to failed wheel bearings I had on the car previously. Also, the noise seemed to almost "switch on" at around 40MPH, which seems odd for failing wheel bearing. Each rear wheel bearing has been replaced less than 30k miles ago, at different times, using the updated taper-roller type. I checked the brakes (it has drum brakes on the rear wheels) and found nothing loose. I did notice a slight binding as the wheel turn part of a circle, but I believe this was an existing condition long before the noise started. It seems to be more pronounced on one side (left-right), but can't tell for sure as turning one wheel causes the other wheel to turn in the opposite direction. Also, it was difficult to localize the cause of binding - hub, axel, differential, can't tell. Replaced the rear differential oil for the heck of it, and found bit of fine metal scraping stuck to the drain plug's magnet. I think that's normal? The oil was last replaced 30k miles ago. Any insight? Thanks for reading. Additional info: The "switching on" of the noise at around 40 MPH isn't quite so abrupt. The noise is audible at below 40 MPH but the volume is faint and the speed-dependent pitch is low so it takes an effort to discern it. At around 40 MPH, it seems the pitch reaches the resonant frequency of the whatever is making the noise such that the noise volume increases and the noise becomes apparent. Past 50 MPH+, the noise volume decreases again as the pitch moves away from the resonnant frequency. * Registered users of the site do not see these ads. Last edited by junkwallah; 11-03-2009 at 01:59 PM.

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11-02-2009, 12:33 PM	#2
Bayley Scooby Newbie Member#: 49763 Join Date: Dec 2003 Chapter/Region: MWSOC Location: Die Auto Stadt Vehicle: 2004 WRX Jetzt mit 25% mehr Motor!	If the noise changes pitch with engine load (acceleration verse deceleration) it is likely something related to the rear ring and pinion gear. A worn ring and pinion set typically makes the loudest noise while "floating". This is when the car is neither accelerating or decelerating but at speed. The metal shavings (which shouldn't be there) are another clue pointing to the differential. A broken or chipped tooth from the carrier gears likely could have found it's way onto the ring and pinion surface initially causing the binding and then eventually the noise. All three R160 failures I've had were a result of this exact failure. Your best option is likely to replace the entire differential with a used one from a junked car. Good news though, the R160 differential has been in use since the late 1960s. Finding replacement units is pretty easy and comparatively inexpensive. Trying to rebuild any differential is always a more expensive option and usually only necessary when trying to find special / custom rear end ratios. I believe you have a 3.54 to 1 ratio, which is one of the most common R160 ratios available. Good Luck!

11-02-2009, 03:54 PM	#3
junkwallah Scooby Newbie Member#: 228149 Join Date: Oct 2009	Bayley, Thanks for the reply. I'd appreciate it if you could share bit more of your experience in regard to the mode of your failed rear differentials, so that I can plan for its possible replacement. Was it pretty abrupt failure since the start of the noise, or was it prolonged and gradual? If gradual, how long was it before you decided the diff needs replacing? Does the noise volume increase? Has it ever completely wear out on you? The noise "switching on" at around 40 MPH, was it also a characteristic of the failures you have encountered? I wonder how much shaving in the differential drain plug is normal. I'd guess some shaving is normal since there is a magnet in the plug, but I can't recall how much I saw the first time I changed the differential oil. Curious also about the typical lifespan of the rear differential. Although I at times have driven it bit harshly (especially with sharp turning), I have not driven it like a street racer, and the entire power train is stock 2.2 liter NA with no modification of any sort. It would be very disappointing to me if the rear differential indeed is worn out at 100k. Thanks for sharing your insight.

11-02-2009, 04:05 PM	#4
junkwallah Scooby Newbie Member#: 228149 Join Date: Oct 2009	Also, the rear differential assembly I found online, a new OEM part, is noted with 3.900 ratio, and is priced at around $750. I wonder if this is the right part, and if I can do better price-wise.