I've been trying to get down to the bottom of this grinding issue for about 3 years now. When i bought the car it was a completely stock 04 STI, 30K on the clock, with stock flywheel, OE clutch - everything on the car was the same as on the day it left the factory. Had no noise whatsoever, completely silent, accelerating or decelerating, no whine, grinding, etc.
Couple of months into the ownership i went on a drive i regularly take with another car, and that included some seriously twisty roads that i've been driving on since i was a kid. That meant driving the s*** out of it, for the first time i've owned it. Almost immediately after that i've noticed a very faint decel noise. I thought that i must've cooked something in the transmission, decided to take a look and change the clutch while i'm at it since the OE was slowly failing as it would cook and start to smell on the slightest of inclines while parking for example. The noise itself was just like the straight-cut gears meshing, also very similar to bearing failure, and it would ONLY occur while decelerating, where the car would yo-yo a few times (meaning that the noise itself would come and go a few times until the transmission "settled" into the position...don't know hot to describe it any better, sorry).I'll just cut with the story and try to line up chronologically what i changed on the car and what effect did that have, if any, on the noise:
- clutch changed to Exedy Stage 1, no change. Exedy was a complete piece of CRAP as it cooked and smelled from new worse than the old OE clutch. Yes, i did resurface the OE flywheel and no, i'm not an idiot, i know how to drive and park a 4WD car on an incline...
- changed to AP Racing Stage 1, no change in noise, although during the time i had that AP clutch the noise was progressively getting worse (some 15K). It did NOT get any worse immediately after the clutch change, it remained the same, so changing these 2 clutches, i'd say, had NO effect on the intensity of the noise.
- changed the tires to Bridgestone RE070, no change. Pressure does NOTHING for the noise intensity, or at least not the type of noise i'm describing.
- took the transmission and all the diffs apart and changed every single bearing, spacer plate, and every serviceable part in them you can think of, including changing the main shaft and the main shaft itself (as it showed very slight sighs of wear at the top). Basically rebuilt them all from scratch. Apart from the very slight traces of wear on the main bearing in the gearbox and slight misalignment in the rear diff, everything was perfectly OK with the transmission. Also, all the tolerances in the gearbox were basically ok and still within maximum allowed tolerances, but as the mechanic told me, the whole gearbox looked "like something was straining the gearbox from all possible directions making it develop tolerances that it should not develop during normal running" - more on that later on. But essentially, all i did made NO DIFFERENCE to the noise level!!! The only change i could put my finger on was a slight change in tone and pitch of the noise but not its intensity.
- during the next 15K or so i've changed all the mounts to GrN, Kartboy shortshifter, rear subframe and diff mounts were changed for Powerflex polyrethane mounts. The noise got progressively worse over time, but i did NOT notice ANY change in noise level or tone "color" immediately after changing any of the mounts.
- at approx. 60K the noise was quite bad. I've checked and rechecked everything i could think of and replaced almost every single part that can be changed in the transmission, apart from changing the whole transmission for a new one. I then upgraded the engine to approx. 380WHP, and fitted a Helix lightweight stainless steel flywheel and double plate organic clutch. At this point i've noticed that a new "layer" of drivetrain noise was added, clearly coming from the clutch as the same type of noise was present when the clutch was engaging. The same, new noise was present when decelerating, adding to the noise that was there before. I was OK with this, since i was aware that some noise had to come from the new flywheel and clutch. This was also a definitive sign that the flywheel+clytch noise and the dreaded drivetrain decel noise are TWO DIFFERENT THINGS! Very similar but NOT necessarily dependant on each other.
- i later went back to a friend of mine, took a gearbox from his own car that did NOT have the noise, and fitted it to my car. The noise was still there, with HIS "quiet" gearbox on my car. I then took the gearbox off, put mine back, and also changed the pilot bearing on top on the main input shaft, the old one was completely shot, which was interesting... While putting the box back and after having changed the pilot bearing, i noticed 2 things: guide pins in the box casing needed to properly center the box on the engine were veeery slightly off center, alowing the box to be mounted while slightly off-center related to the engine. That in turn would explain why the box was slightly out of tolerances like something was straining it from all directions - something was, the engine, it was not centered properly. That, in turn, also destroyed the pilot bearing. And guess what, after properly centering the box and changing the pilot bearing, the noise level and tone decreased and changed DRAMATICALLY. It did not disappear however, and after a while came back again and started increasing in intensity. Hence, from all above, my conclusions are as follows:
0. THIS NOISE IS NOT AND CAN NOT BE NORMAL, AND CAN NOT BE ACCEPTED AS SUCH! There must be something causing it, either wear and tear or a fault slowly developing since on many cars it was not present from new. If the problem is there and we can't find it, that does not make it acceptable or NORMAL, just persistent and hard to find...
1. Engine and gearbox mounts, bushings etc. have no effect whatsoever on the deceleration whining noise we all seem to have a problem with. They might add to other types of NVH levels otherwise not present with OE parts, but not to this particular noise.
2. Same goes for the tires. Btw, even if tires WERE the cause, i do NOT want the noise level to dictate the pressure in the tires, that's kind of stupid, kind of reverse logic to me, i might just turn up the stereo to cure the problem according to that same principle.
3. Flywheel and clutch have NOTHING TO DO with the noise. They create different kind of noise, but unrelated to this deceleration grind and whine. Many people with OE parts (me included) have the same problem, as well as people with Exedy, ACT, Helix and many other clutches, flywheels, etc. leading to the conclusion that there is something else at work here.
4. No mechanical failure or component wear in the gearbox or diffs or any other transmission component are the cause. Same goes for diff mounts (no problem there), rattling heatshields (i don't have one), driveshafts (no play), propshafts (tried a carbon one, no help), etc.
5. The ONLY 2 things I can bring into any sort of meaningful connection with the issue are: A) improperly centered gearbox after it has been taken down for any sort of intervention, and B) failed or failing pilot bearing on top of the input shaft. Failed pilot bearing (very small and weak, btwÖ) will cause excessive play in the shaft and cause excessive tolerances in the box after a while, while improperly centered box will also make this bearing fail and also cause minute tolerances in all the gearbox components - not enough to say that any single component is bad, but enough to cause significant play in the gearbox while not causing any real damage. Whether the bearing would fail on its own I canít tell, but one problem definitely causes the other. Which causes which I donít know, not important really. People usually take the box down for clutch change, donít center it properly and put it back, later blaiming the clutch for excessive noise. Or they center it properly, but dislodge the failing pilot bearing from its place making it fail completely, donít change it and have the same problem. The centering pins on my car might already be too shaken apart, so it canít be centered properly anymore anyway, and I didnít bother to change the new pilot bearing when trying out the new box. If I did, I guess the noise would be either significantly reduced or gone.
It's always a bit annoying when i read excuses and problem "solutions" that read something like "it's a very heavy duty box and needs to grind a bit because it's so strong...". First of all, it is strong but not THAT strong to produce that kind of racket. Second, strong boxes do not necessarily have to be noisy. Third, if they were not noisy when new, some play and backlash MUST have developed in the meantime, which is NOT normal - tolerable and harmless because the box is strong, but not normal. Trick is only to determine where the excessive play is located and why it happens and occurs in the first place.
From all of the above Iíd suggest that next time you take the box down, check if all the guiding pins for the box are in place, not pressed in or distorted. If they are, the box will slide in place and appear ok, but will NOT be centered properly. Also, make the pilot bearing change MANDATORY each time you take the box down. I know I will and I bet the noise will be reduced again.
Iít be interesting to know if any of you have any effect on the noise with this.