chokboywonda

Hi guys,
I wonder if any of you have any ideas of what this horrific noise is coming from the rear left area of my car. It is not there often but when it comes it screams! I have heard it only 4 times but when i try to get it to happen again it won't. It sounds like someone is sawing or grinding metal. It happens at around 70 - 80 mph and if I take my foot off the gas and let the car slow down by itself it gradually increases. It is definitely metal on metal and has appeared around 4 times over the past 6 months. I do not think it is bearings and have thought it could be a brake guard plate rubbing or something trapped in there but i can't see anything obvious. I haven't taken it in to my mechanic yet as I can't seem to get the noise to happen but I need an MOT soon so I will ask him then. It happened yesterday just as I came off the motorway and stopped at my house. I got my Mrs. to drive it and I drove next to her in my A4 with the window open so i could record the noise and try to pin point where it is coming from but it would not make the noise.
It sounds like this at 2.58 but louder



Thanks

MP4.2+6.0

I'll skip repeating the ones you already have thought of, which are possibles.

Some more, several of which you can also inspect for:

1. Bearing bolt (NOT the bearing, the retaining bolts) that has backed out and is starting to grind on the CV joint area. Had exactly this happen on my 2000 A6 4.2 with the cartridge bearing set up like the D3. Dealer had replaced a bearing and obviously didn't get the torque right, or no LockTite or something. On mine, once it hit, it was a terrible racket and didn't let up. Had to tow car home. As soon as I jacked up wheel and spun it, obvious something was hitting horrifically. Lots of metal shavings once I looked in the right place. Harder to explain though on yours how it might be sporadic, unless parts are heating up at times or something. The inspection is jack up wheel and spin looking for noise. Assuming it won't be that obvious, look for metal shavings/particles around the outer CV joint right where it goes into the hub area.

2. Hanging up brake caliper that has let pads run off angle, particularly on the inside. If it goes off angle, it may not yet throw the rear worn pads signal, but could grind. I've seen it on older rear Audi single piston set ups, which are like 99% of Audis out there, D3's included. Remove wheel and visually inspect pad area, especially inner. Would be happening only sporadically if pad can move around some due to caliper, shim or backing plate related wear or issues, and then it may periodically hang up/jam off angle.

3. Physical ABS related failure--either the sensor hitting the toothed gear on the CV joint, or the toothed wheel coming slightly off the CV joint. May be hard to see. Perhaps pull sensor and see if any metal shavings have magnetized themselves onto it. I've heard/read a few posts over the years about that one.

4. Rear diff bearing or something like that. Ugly. There is a U.S. TSB saying earlier ones for W12 and S8 are weaker and get noisy. Maybe a similar design for higher torque 4.2TDI's too? The older ones are blackish or grey (painted or dark metal when new); the new and supposedly improved design is more a silver metal color apparently. The noise though is apparently gears. They rumble at speed when you get that, but it is typically consistent, not erratic or occasional.

5. Also check the obvious--wheel bolt tightness and fit. If by wild chance you are using a bolt on spacer like the H&R's DRAs or even a normal one, if the bolts go too deep in they end up bottoming and setting up a grinding issue. BTDT on rear of my W12 with one spacer set when I changed from older OE 20's to newer OE 20's and apparently the recess for the bolt head to hub face was just marginally thinner and just enough to make it no longer clear. Again, hard to explain though why this would not be constant except weird differential expansion issues or something. I suppose if married up to a wheel bearing starting to get some axial play in it, it could be a weird corner case.

chokboywonda

Thanks
I got the noise yesterday and my dad was there to listen to it when I slowed down. He thinks it maybe the brake sheild as it was def coming from the wheel. I will take a look tomorrow for sure.

richard-tx

Dust shields can make a horrible amount of racket. Lightly pushing or pulling on it while someone turns the wheel will usually confirm that theory. If so, giving it a pull to slightly bend it away from the rotor usually fixes it.