[Biturbos4] A whine from the rear

Chris Newbold chris at newbold.org
Fri Jul 14 08:54:56 EDT 2006

On Thu, 2006-07-13 at 11:46 -0500, Keman wrote:

> Keep in mind that the rear diff only spins at around 850 rpm on the output 
> side and 3300 rpm on the input at 60 mph. When rear diffs go, they make a 
> very specific HOWL. Which unfortunately does change with throttle as you 
> describe.

The noise is something like WHOOOOOO... At first I thought I could be a
fuel pump, but the pitch seemed a little low and the connection to
throttle inputs pretty much rules that out.

> What's the pitch of the noise? Have you had someone sit in the back to see 
> if the noise origination sounds elsewhere?

That's my next step; it's certainly coming from behind the front seats,
but it's difficult to localize it better by myself.

The only other factiod I forgot to mention is that it gets worse as the
car warms up. I have a ~20 mile highway commute to work, and the noise
is not reproducible when I first get up to speed. After 15-20 minutes at
speed, though, then the noise starts getting more prominent and is very
reproducible. Once things are fully warmed up, you can start to hear it
more faintly even at lower speeds, 55-65MPH.

> It never hurts to drain the diff and refill it. See what comes out.

I'm going to take it in to the dealer soon and see if I can reproduce
the noise for them. I've still got 10 months of Audi Advantage warranty,
so if the diff is going to blow I want to set the stage for them to pick
up the bill.

Thanks for the thoughts.

2001.5 S4 Avant 6spd

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