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Heres whats been done:
New pss9 suspension - using custom springs on front as spring rate/ride height adjustment was just not good enough using pss9 springs (custom front springs on front allowed better height adjustment with better spring rate which removed the awful road surface feedback through steering wheel from pss9 springs)
New suspension arms top and bottom front & rear
new suspension bushes all round
Engine mounts are all good
Gearbox mounts are good
rear diff/prop shaft new
Front diff Drive shaft housing new,
Hotchkins arbs front rear anti role bars
Wheel Bearings all new
New Brake pedal bush - which fixed the brake pedal rattle (the one that make you think the subframe is falling apart)
interestingly the rattle that is left occurs over similar road surfaces
Miltek decat non res, 3in dp - exhaust mounts all feel tight and good condition.
Basically i have one rattle left that I just cannot find / identify...

Local Motor sport specialist found drivers side headlight loose - a contribution to the rattles, but not the rattle!
They also found inter coolers not sitting in theirs mounts properly, again adding to the knock noises over bumps, but nothing significant.
Unit20 who did bulk of work never found the rattle, if anything the pss9 suspension / arms/ bushes made It worse /more noticible. The reverse of what i was expecting. Had to use local specialist to get pss9 suspension Setup something close oem feel, but better than drc. sorted, sort rattles/knocks.
It feels more gearbox, drivers floor pan area, and is more a knock, followed by another resonated knock caused by the car being shunted from left to right. if it was a loose glove box, or car phone handsfree speaker that was badly fitted, its that kind of noise, but feels under car, not from the within cabin.

Thoughts??

Fuel tank straps tight?
Battery tight in boot?
Cd changer unit tight?
Amp tight?
You need to know where about's the noise is coming from first. Have you had someone sitting in the rear to help identify where the noise is coming from?

Loose steering rack?

Have you had your TCU modified? The screws are a pain to get in and it's not impossible that some lazy grease monkey didn't bother tightening them.

Had your exhaust position checked?

Could be something broken inside the exhaust manifold. Some other Audis (B5 S4 springs to mind) suffer from this quite a lot but it's rarer on the RS6. Can happen though. Best to get it checked before a chunk breaks off and kills your turbo.

Are the struts properly bolted in place at the top mounts?

Steering rack has been checked by Unit 20, and local Motorsport - no play.
Exhaust position is all ok, unit 20 installed the full miltek system when engine/gearbox were removed - local Motorsport company not suggested a problem with alignment either. Not sure on what can break inside the manifold, I don't think it's that kind of noise, more drivers floor pan, gearbox area. Not sure if its metal noise, more knock/rattle under extreme bumpy surface, also at slow speed.
My only thought is that exhaust might be catching on something when car is really being wacked by bad road surface, typical left to right vigorous motion. Otherwise it's fine. Annoying though. Struts all fine too.
My Motorsport chap thinks it might be torque converter, I was sceptical, is that what you mean by TCU? My understanding is that Audi either replaced or repaired gearbox, with previous owner but have no record of it in his invoice history. He just told me it had been done under warranty not long after he bought it. I can check with Sheffield Audi?
Cd changer has been removed, luggage guard (looks like dog guard) also removed. It was doing noise before these were removed (weight saving). Battery is tight, not sure on fuel tank, but my feeling is that this type of noise would be noticeably from rear instead of upfront.

TCU sounds interesting - how would this rattle if screws aren't tight? What's it do and where is it?

The tcu isn't screwed in place , it's held in place by studs on the floor and the carpet keeps it from moving about ..

Your motorsport chap thinks it may be the verter . I would possibly stop using them if he thinks that mate. The verter ifit was loose would rattle pretty much all the time and would be noticeable when revving the car.
Have your seats been removed? Does the rattle happen someone in the passenger seat?
TCU could be a suspect as Shopp mentions.
You say you have 3in downpipes, are these close to the chassis?
Did you have the new diff, diff housing and propshaft due to this noise?

grizz wrote:The tcu isn't screwed in place , it's held in place by studs on the floor and the carpet keeps it from moving about ..

My TCU as been in and out more often than John Holmes' todger and if those aren't screws that I undid then I need to re-evaluate my understanding of mechanical things...

/edit: Looks like a screw to me...

Maybe the prop shaft center bearing hanger is loose?

Heat shield somewhere? They can make nasty noises.

I can't for a second imagine it being the TC.

Shoppinit wrote:Maybe the prop shaft center bearing hanger is loose?

Foook me, that prop shaft bearing thingi failed on a friend's BMW on the weekend, it felt like a flat tyre, or driving over rumble strips at. It was only £40 for a new one, and he fitted it himself in four hours. Imagine it would be a lot more on the six.

chunky79 wrote: Have you had someone sitting in the rear to help identify where the noise is coming from?

Seems the obvious starting point

Shoppinit wrote:

grizz wrote:The tcu isn't screwed in place , it's held in place by studs on the floor and the carpet keeps it from moving about ..

My TCU as been in and out more often than John Holmes' todger and if those aren't screws that I undid then I need to re-evaluate my understanding of mechanical things...

/edit: Looks like a screw to me...

I meant the box it self ..

You lot have all been helpful and technical..

My input on resolving this..

Money in the ash tray, it annoys me loads.

swannny wrote:You lot have all been helpful and technical..

My input on resolving this..

Money in the ash tray, it annoys me loads.

Money in the ash tray? You not paid Grizz yet then?

If I go over speed bumps, at slow speed, it also makes the noise. Ive had drop link problems on other cars before and the noise is similar, but everything has been replaced. No play or slack or worn bushes. Both unit 20 and other chap are saying the same thing.
TCU looks like a good one to rule out especially floor bolts holding it in, but given its location and material, not so sure. When going over speed bump, noise and feel of clunk/knock/rattle (a bit of each) seems to originate from gearbox, feet, back of engine area. Almost subframe area.

anything in your front and rear arm rests , or a empty coke can under the seats