On deceleration I get a soft "tonk" and then on acceleration another soft "tonk". There's also a low bassy vibration. I assume bad U-joints. Any alternate diagnosis out there?

So, best way to confirm this suspicion? Also, if anyone has brand recommendations or any details I should know when ordering that would be great!

I've replaced these on my XJ6, any differences here in the XJS?

 

How is the transmission mount, IIRC Greg had a vibration on decel that turned out to be the trans mount.

Replacing the U joints is not difficult. When I did mine I used Corvette solid joints which are much stronger. To get them out and in I used a G clamp and a couple of appropriately sized sockets.

 

Trans mount is new, snug. Definately sounds like it's from the right passenger axle area.

I have a large C clamp and appropriate sockets all zip tied together from the last job.

What model 'vette did you choose them for?

 

If a u-joint is worn enough to clunk then there will probably discernible free play if you lever against it. Plus, that much wear is typically preceded by a vibration or shudder. But....none of this is carved in stone.

Also check for the exhaust contacting the underbody

Cheers
DD

 

It was too long ago so I can not remember but I do remember the U joints were Hardy Spicer.

Doug had a good point with the exhaust, mine was touching on the trans X member on a left turn, sounded like a driveline issue but once I relocated the exhaust noise was gone.

 

Or possibly an engine mount.
Can you replicate the sound when the car is stationary?

 

Is the exhaust over axle pipe mounted correctly?

 

A few ideas to add to the others:

• Exhaust fouling the cage is my favourite of all the above suggestions
• Check the radius arm bushes and fixing bolts both ends
• Check the lower inner fulcrum brackets are still tightly bolted to the diff. (A search on "dog bone" should reveal a thread on this matter)
• Check rear shock absorber fixings are tight
• Check prop shaft bolts are tight BOTH ends
• Check drive shaft bolts are tight both ends
• Check all cage to chassis bolts are tight
• If all negative, check the diff is secured to the cage tightly (requires axle to be dropped as fixings are at the cage top

Normally when UJs fail you get either a crackling noise or a real clonk.

 

That's a pretty long list! Thanks to everyone with suggestions!

I'll start underneath with a microphone on the rear axle to see if I can get a good recording. I'll try in place shifting from forward to reverse and also have a look at the exhaust routing.