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Old 02-22-2012, 09:00 AM   #1
Big-E
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2006 WRX Sedan SGM
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Default Engine sounds like a diesel or rocks rolling around after cold start

When I start the car in the morning, the engine of course goes into warm-up high-idle.

I let the engine run at this speed for about 30-seconds and then lightly tap the gas pedal. Similar to the carburetors of the past, the idle kicks down to a more moderate speed.

Sometimes when I lightly tap the gas pedal to drop the idle down, I can hear the engine 'rumble' when I am doing so for that split-second.

Once the idle has dropped, there is no such 'rumble'. But, after about a few more to lets say 30-seconds after the idle has dropped, I will then begin to drive. When I do so, this 'rumble' is pronounced when lightly feathering the gas pedal. It does not matter if I am going forward or backward - the direction of motion (drivetrain) is not the cause of this 'rumble'.

There are occasions where I will feather the gas pedal after the high idle has dropped but before I drive just to see if this 'rumble sound' is there and it is.

But, whether the gas is feathered with or without driving, the 'rumble' will always abruptly stop and the engine makes no such noises thereafter.

Based on the information I could find, could this be the AVCS or a timing-belt tensioner?

Another way to describe this 'rumble' is to equate it to an emissions valve failing and allowing exhaust to recycle. I do not have the air-feed system anymore and have block-off plates on each head. But, when the air-feed valves were in place, the driver's side valve, during the winter, was freezing due to water accumulation. As a result, this secondary valve would stick open until it warmed up, causing an exhaust rumble back-feed through it.

This engine 'rumble' sound is similar to that. This is the best way I can describe it.

The engine is stock, but the car is tuned and modded and so I am my own warranty, which is not at all a problem.

Any thoughts?

Thank you for your time.
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Last edited by Big-E; 02-22-2012 at 10:46 PM.
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Old 02-25-2012, 09:19 AM   #2
REDrexsti
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So it makes the sound only when it's cold? Or also on warm starts as well?
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Old 02-25-2012, 01:28 PM   #3
Tdaz250
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How cold is it outside when this happens?
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Old 02-25-2012, 09:36 PM   #4
Big-E
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Around freezing to 40 or 45 degrees.

Today I went to work and the car sat for about five hours. I started it and no sound.

It's as if something is sticking and then instantly works and the noise goes away.

I'll probably pull the covers to the timing belt, put everything back in place and then start it the next morning and look at the timing belt tensioner and idler pulleys. Maybe next weekend if I have time.
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Old 02-28-2012, 12:32 PM   #5
bangedlegacywagon97
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Does it consume any oil? It might be stuck rings or just regular old piston slap
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