r7ehw

Hi,

I bought a 2.7T allroad 2001 at the beginning of this month which is a very tidy 2 owner from new car with an inch thick pile of main dealer receipts and full history from Audi. I really like the car but there is a but... on test drive there was a a knocking from the rear of the car usually coming in when we went over a bump. The knock is intermittent. I was told it was from the rear brakes and that it would go. It hasn't and I now know its not from the brakes but from the air suspension.

The noise occurs on most journeys at some stage but only usually once and will either stop of its own accord or will stop when the car comes to a halt or when I push the up or down buttons on the dash. The knocking sound is like a repeated banging, quite loud and like a piston hitting something, maybe like an ABS system operating although as I have said its not the brakes.

The car has been to see a VW/Audi specialist through the selling dealer and they are at a loss and said it could be a number of things and that I could be in for big money with it. Maybe £2,000. The dealer has offered me a refund on the car which I am minded to take however I am a bit gutted to be fair and don't really want to be car shopping again.

Your comments and advice please?

G0to60

I bet it is your air suspension compressor. What happens is the piston ring wears down and causes slop and you usually have a hard time raising the car. The compressor also comes on occasionally as the air suspension needs adjustment.

It looks like you have two options. The cheaper solution is you could rebuild the compressor. There is a guy in your part of the world (look up bagpipingandy on here and other sites) that makes a rebuild kit for a good price. It was about $75 plus shipping so yours will be cheaper. It took be a couple of hours to rebuild it but I took my time. If there is damage to the piston or the cylinder then this option probably won't work.

The other option is to replace the compressor. New ones go for around $400 in the US but may be more or less expensive over there. This is also easy to change out and will take a couple of hours.

r7ehw

Thank you GOto60. I have watched Bagpipe Andy do his stuff on you tube. Not frightened of a couple of hours work but its the "what ifs?". I might suggest my dealer gives it a try as if it does fix it I am happy to take the car.

amiram

Check if there is no loose objects in the spare tire chamber. Check the rear stabilizer links

r7ehw

Thanks. It is not a loose object in the tyre well as is like a mechanical noise like a piston going bang bang bang bang etc.

Rear stabilizer links?

Worldwidebeagle

having rebuilt an AR compressor, it does not sound like the issue. When the compressor is worn, it makes a racket when operational - i.e. building pressure or trying to build pressure. When the compressor is off, I cannot image it making ANY noise. The compressor noise when running has nothign to do with bumps.

Amiram's suggestion about the rear stabilzer links is good. the rear stabilizer bar has links and well as bushings. if either a rubber bushing in the link or the bushing holding the bar to frame is broken/disintegrated, then it could cause a knocking. But that knocking is more like bolts in a tin can. Same with upper control arms (which rarely ever go on rears).

You might give my friend Andy Fulton (Bagpipe Andy) an email - he is a whiz on the suspension system. He could tell you instantly if the knocking could be related the suspension compressor.

bagpipingandy

I think it is the compressor but not the piston ring, check it is sitting on its spring mounts correctly, if not it bill bang against the antiroll bar and give you a strong thudding now and again, also the compressor stops every time the car stops moving so it does sound compressor related but more the fitment of the compressor,

it should sit nicely on its 3 springs and bounce on them not touching the cars body, go have a look at the comopressor to see how it sits

best regards

Andy

r7ehw

Hi Andy,

I have at last got around to getting the compressor off the car and one of the things I did do was to change the piston ring with one of your kits. I also had a huge problem actually getting one of the alloy mounts off the car but my local garage gave me a hand with that when it was in for a service.

So I have removed and refitted and put in place a new piston ring. Well the noise is still loud but different tone clearly changing the ring has made a difference but the noise is still very unpleasant and my wife thinks the car is about to disintegrate every time she goes in it. Is it possible that the compressor is worn in another way and needs replacing? the thing is it does work and nothing is wrong with operation of the car. Its all about the noise.

Ideas suggestions for my next step welcome.

Thanks in advance.

Russell

Metronet

I had a super noisy pump that lost 3 out of the 4 rivets security the side panel to the body of the pump motor. It worked for a while after I drilled them out and replaced them with bolts and nuts. I eventually ended up replacing it with a Mercedes unit from ebay and it is much much quieter.

Not exactly sure what happened to the original pump that lost 3 rivets but I guess the damage had been done after leaking air for a while. Who knows how long the previous owner been running like that.

Torsti Tenhunen

Check if you have a non-Allroad sway bar. People replace Allroad sway bars with stiffer standard sway bars from S6 and RS6. Their curve at the compressor is different and the compressor knocks to it when operating or on bumps.

I covered my RS6 sway with polyurethane tube at the compressor.

Audi has one stiffer OEM sway bar that has a special curve for the compressor. So does Hotchkis.