scottdbutcher

Hi guys! I'm newly registered with the forum, I have been lurking for a long time. Your combined knowledge base is quite impressive! I have used numerous DIY's to help me on my repairs to my car and they are always spot on! I have been searching for an answer to my current problem for a while with not much in the way of a concrete answer, hopefully someone here can shed some light on my situation!

Ok here is it. 1998 A4 B5 Quattro 2.8l V6 5 speed 183K. Started with the rear wheel bearings, they were whirring away, so they were replaced. After replacing those, I still had noise that sounded bad bearings, so I did both the front ones also.

Now that they are done, I still have a constant noise under speed. You can really start to hear it at 15-20mph and becomes an annoying drone around highway speeds. Its a very subtle whine, not grinding or whirring like the bad bearings were. Its constant depending on road speed. Disengaging the clutch and shifting have no effect on the noise. When I did the rear bearings it was clear the seals on the rear diff were weeping. I added a quart of mobil one 75/90w GL5 and topped it off, as I planed on doing an output shaft seal change soon and didnt want to flush it twice. The noise is definitely coming from the center/rear of the car, which leads me to believe the input bearing in the diff is bad? Or perhaps a carrier bearing in the driveshaft? CV's all look fine also.

I have dumped alot of money into this car, and would rather not dump more in or go though the hassle of changing the rear diff if it may be something I overlooked, something simpler. Does anybody have any ideas or has had a similar situation? Also if it is the diff, I have access to one from a 2000 1.8t quattro. Would that work with my 2.8l? Both cars are 5 speed. I cant find a solid answer on that either. Does anyone have a DIY to change the rear diff? Im sure its straight forward, but it never hurts to read up on what you are in for before you get involved! Thanks in advance for the help!!!

turbo510

I have a FWD car, but if you really had to add a whole quart of gear oil to the rear diff, I'd say you've found the source of noise. I had an Explorer that had rear-end gear whine, really annoying. As to putting in the 1.8T's unit, be absolutely sure the ratios are the same.

4Driver4

Agreed...almost. One quart low is zero oil in the rear diff. Bad. However, I'm surprised that you didn't mention checking either of the other two diffs. Why not start by topping all of them off?

scottdbutcher

Right after I discovered that the rear diff was low, I immediately flushed and replaced the fluid in the front trans axle/transmission. While the fluid level seemed ok, the fluid was extremely viscous and dirty. The trans definitely benefited from the fluid swap. Seems to shift much nicer now. So all that is fine.

4Driver4

Is this car stick or auto? If slushbox, which fluid in the trans are you referring to? There are three chambers.

scottdbutcher

Cars stick, was covered in OP. The transmission and front gearbox share fluid, so there is one drain, and one fill for the front transmission/gearbox. So unless I have drained the headlight fluid by accident its been addressed.

4Driver4

Ah, yes. I see that now. Got thrown off by the FWD response. Carry on...

scottdbutcher

No problem! Have you ever seen/heard anything like this? All I read seems to point to the input bearing in the rear diff. I just hate to swap the whole thing and find that wasnt it! I mean it was significantly low on fluid, I suppose that bearing (or all of them) could have toasted... but its slowly getting worse...

Any idea on compatibility of 1.8t rear diff to a 2.8?

4Driver4

Wheel bearing is still the most likely source of the noise. Does the sound change slightly if you swerve gently while driving?

scottdbutcher

Nope. Not at all. Only changes with speed. Gets louder. I dont think its a wheel bearing, as they have all been changed in the last 500 miles, and I purchased quality bearings from ECS.