silverd2

Yes..to simply grabbing each halfshaft and twisting. There should be no radial play between shaft and CV joint...solid direct feel.
There is also a CV joint at each end of the central drive shaft.
But as I've stated before, nothing about this is a typical (that I've ever seen or heard) of any CV joint.
Have you had a good look at your front torque support engine mount(visible under hood, between motor and radiator, passenger side), for missing/broken rubber or loose/broken bolt?
If it's broken or loose, it will be very apparent with a helper to hold foot on brake and shift into reverse with engine idling (and maybe give a little gas). In reverse, if it's broken, loose or separating, it will open up or move.

aduggs

Thanks for confirming that there should be no radial play in the CVs (some axial play is normal?)

I will also check the front engine mount, but I am fairly certain this is not the problem. Good to check anyway.

I can definitely hear the clunking coming from the rear of the drive train. When coasting from 10 MPH down to 5 MPG (I have the door open just to get a good listen), there is definitely an audible metallic clunk coming from the rear.

This is quite confusing to me, since I know there should be some play in the drivetrain (driveshaft to diff). Therefore there should never be a torque reversal in the drivetrain otherwise there will necessarily be some backlash. Is it possible that the shift points (down from 2nd to 1st gear) of the trans are off causing a torque reversal in the drivetrain? This is the only thing I can think of.

Thanks for the help.

pocketchange

I've never had a problem in diff without metal migration (a good flashlight should help spot any metal).. Warm it up and change it, to be positive.
Having a magnetic drain plug would be handy.

Remove and inspect the drive shaft assembly for excessive ware.

Have you been able to get on a lift and cycle the transmission while holding the brakes and observing the driveline?

99% of the time, look to the rear CV going away [unless] it has had recent service.

I would be hesitant to drive until the problem was discovered [and repaired..].

silverd2

Axial play is, in part, defined a little differently in CV's, compared to a traditional (wheel) bearing. CV's of course have off-perpendicular swivel ("play") but NO off-parallel play...shaft up/down play. Outer CV joints should have NO in/out play (part of defined traditional axial play).
However, inner CV's (triple roller joints inside up front) and CV's at each end of central dive-shaft are designed to have a lot of in/out movement to allow changed length of shaft during up/down suspension travel (and motor/diff movement)...totally different design, in that respect, than outer's. But again, NO off-parallel play (up/down) in shaft to CV connection in any of the joints.

While looking at rear suspension, besides the obvious (like broken/disconnected sway bar bushings or suspension arm ends, etc), I would pay close attention to the 2 diff carrier cross-member rubber bushings (individually replaceable)...I've heard of these wearing out/breaking and allowing radial movement of the entire rear diff.

silverd2

Please explain..for academic purposes

The central drive-shaft? ...quite involved removal....and check for what "wear"? Are you talking radial or up/down axial play in the CV's and/or the shaft's central universal pivot (plus it's center mount bearing)?

Most (all) of this can be checked (preferable) with thew drive-shaft installed. The center uni-pivot is tougher to get to, but central drive-shaft end CV's can be checked as is..installed.

It's not unusual (normal) to have lots of radial play in the diffs themselves, but NONE in the joints themselves. Also not unusual for the drive flanges (CV's bolt to) on the central diff and all 3 in rear diff, to have just a "click" of up/down play (NOT movement...just a click...grabbing inner and central shaft CV's and moving the flange in the diff)..BUT NOT in the CV itself connected to any of these flanges...the CV's are much tighter tolerance than the flange's connection to the diffs, just by the slightly "looser" feel of diffs, by nature of their design.

aduggs

With AirCon on - no clunking; with Econ on - then clunking. This really does not make much sense to me why having it in Econ mode would stop the clunking. All I can think of is that with the AirCon on, the engine does idle at a slightly higher RPM (around 700 RPM). With Econ mode, the engine idles at 500 RPM (which seems low to me). The low idle could explain why the shift points are off causing the gear backlash and clunking

If someone could explain this, they deserve a pat on the back.

aduggs

My low speed clunking problem has been mostly resolved. Now I can barely feel it, almost unnoticeable. A8 shifts as smooth as silk again! And I just past 150k miles yesterday - this car is amazing.

Solved with a combination of:

1. Reset throttle body with Vag-com
2. Flushed ATF fluid (it was low due to a slow leak past the pan gasket)
3. Disconnected battery to reset shift points.

My guess was that the main culprit was low ATF. If you have this problem, check your fluid level.