Tow car problems

[OMS]

I've got a 99 audi A4 2.5 TDI Quattro Avant with 88k on it.

I have owned this car for a year, mainly bought it to tow a race car and it's been brilliant in every way... anyway about 5 months ago I noticed that at around 70mph there is a rotational rumble coming from the rear, it get's progressively quicker as you speed up. I took the rear wheels off and noticed that a read spring had broken. I fixed that and thought my problem would have gone away... nope. The wierd thing is when you brake hard at speed (70 - 80mph) the rumble turns into a shake. I thought it was the rear brakes, nope they got replaced recently. I thought it was tires nope they're brand new and I've swapped wheels to see if it was a buckled wheel..nope it's still there.

My other thoughts are;

Wheel bearing but that doesn't explain it getting worse under braking.
Drive shaft CV joint on it's way out but I've just jacked the car up and had a look and there's no play there.

Any thoughts?

Thanks in advance

[Trooper2]

I had something similar on my old Isuzu and thought it was the brakes as well, it ended up being the ring and pinion gears in the rear differential. Try grabbing the yoke that comes out of the dif, push it up and down and see if there is any play, there should not be any. I put a dial gauge on mine and got .040 I couldn't see the play but I could feel it.

[OMS]

Trooper2 said:

I had something similar on my old Isuzu and thought it was the brakes as well, it ended up being the ring and pinion gears in the rear differential. Try grabbing the yoke that comes out of the dif, push it up and down and see if there is any play, there should not be any. I put a dial gauge on mine and got .040 I couldn't see the play but I could feel it.

Thanks Trooper2, I have a sneaky suspicion that it might be the diff but at 88k I didn't expect it, especially of an audi that's only 6 years old. It's leaking from the main prop shaft seal and one of the drive shaft seals - not enough to leave drops on the garage floot but noticable. In your experience what's likely to happen if I leave it and plead ignorance?

[Trooper2]

Have you checked the fluid level in the leaking dif lately? If it's leaking and low that could cause some noise for sure. It sound however that you are beyond sounds and some damage has already been done, somewhere. When you get the chance try to feel for play and if there is none check the fluid level for safety sake. If you don't have play at the yoke and fluid level is OK then that would tend to rule out the rear dif which is probably the most expensive thing in the rear of the car. If you do find it is the rear dif gears they will only get worse. Chunks will start to flake off of the gear teeth; then the teeth will break; due to the backlash adjustment being too great and could eventually strand you some place.