Did much searching, here's my conclusions, any additional insight appreciated.

Trying to ID if it's one of the following:

1) Rear end
2) Chattering clutch on locking TC
3) Tranny or part of it.

I have a 98 A4 all stock except lid, and had a tranny die. I had a tranny swap with low mileage used A4 tranny, and while the tranny was out I figured now's the time to have them throw in a high stall (Vig 3600) since that's the biggest bang for the buck ET-wise. Car has over 150K miles and is my daily driver when it's not snowing. Local Camaro speed shop did the work, they do many Camaros so I believe it's in correctly.

It's all working well except I have this very loud noise that happens ONLY while TC is locked and ONLY at extremely low RPM (under 40mph), I think like 1800 RPMs or something, in BOTH 3rd and 4th gears, it happens. Sound goes away if tach rises above say 2000 RPMs. Sounds exactly like a garbage disposal and is very loud. Resonates through tranny and rear, hard to hear where it's coming from, sounds like right under cupholder in center console if I had to pick. It is sort of a "rotational" noise goes up and down with speed, so I don't think it's exhaust rattle or anything like that.No sign of TC slipping while locked, RPMs rock solid until unlock threshhold is reached while accelerating. All shifts seem fine, tranny temp is always 130 to 170 and fluid is clean and right on full line.

The used A4 tranny came from a non-wrecked A4-to-M6 swap vehicle and it works great, I don't think the tranny is bad, but possible. It was fine when in the previous car. The rear seems kinda loose and sloppy (original 150K rear). Noise was NOT present prior to tranny swap with the old 150k dead tranny (sun shield gave out on the old one).

Here is my checklist for my mechanic, PLEASE add to this if you would, and feel free to comment.

1) Drop shaft and rotate yokes 180 degrees, listen for changes. (suggestion from friend)
2) Look for cracks/problems with the bolt-on tailpiece that often breaks on these trannys.
3) Verify U-joints again (they were quickly checked when tranny was swapped).
4) Change diff fluid, see if tone/sound changes (maybe goes away...?)

Anyone else have anything I can do to identify where this horrible noise is coming from? Could it be slop in the rear that's only now showing up because the new tranny's tighter? Can I put in some thick diff fluid just to run a few miles as a test to see if it's rear? Is there some way to tell if maybe it's the locking TC clutch chattering (zero slip detected)? If you have any ideas of things to try to help ID where this problem is I'd appreciate knowing, taking it to mechanic hopefully next week weather permitting.

Thanks all,

Mark from Pittsburgh

 

Well Mark that's some good info there, thanks for the detail... if those guys who did the job are worth their salt.. and I'm sure they are... then they'll be able to drive it, verify the complaint and then determine the best course of action. It is highly possible that the noise can be observed while the car is up in the air on a rack also. If so, then it won't be too awfully elusive.

Vig makes a fine converter, but it is possible that a component inside it failed... a bearing, sprag or something else. It is not unheard of.

I recommend taking it back to the shop that did the install, ask them to ride with you so they know exactly what they are looking for and let them take it from there.

Hope that helps.

g

 

Because of the limited RPM range of the noise, You might also consider and instalation issue, Exhaust resonace it can sound like its coming from everywhere, I once loked allover for a resonate sound like that only to find one of the rubber exhaust hangers had come loose during the install something to consider