My daily-driver 1996 VDP took a hit from a tree branch in last week's windstorm - obliterated the driver's door.
So, while tracking down a replament driver's door for that car, I found the twin-brother of that car on CL for a song - BOUGHT IT!
It's a 1996 VDP, with Harmon Karden upgrade sound, heated seats, much nicer clear-coat than my other, peeling, VDP.
I'm happy!
B U T . . .
When motoring along at 60mph, if I simply let off the gas and coast down in speed, there is a rather LOUD RUMBLE from 55 to 45.
The rumble seems to be be body-panel type noise, or interior fitment rumbles. (reminiscent of a 49 Dodge Power Wagon at any speed)

Even if I am going uphill a bit, it still rumbles (but there must be 'braking' going on from the engine to the drive wheels).
In any gear, but not in neutral.
Using the brakes does not reduce or increase the rumble.

Rear axle u-joints were freshly greased up.
Flex disks look like they should.
No visible damage to any components.

I'm stumped.
Any thoughts?
Thanks!

 

There is the thing with the output bearings on the differential casting body but I have heard it myself on another X300 as a ticking noise

side dependent unless both sides eventually fail

Exhaust rear muffler mount rubber , the mount can be removed but hard

There is a list of rattles in the later sections of the TSB list

JagRepair.com - Jaguar Repair Information Resource

 

Check the driveshaft center bearing for wear

 

The power off onset of the issue would suggest drivetrain, most likely differential. Do you have a particularly high mileage? Any signs of oil leakage at the diff?
It could indeed be some trim vibrating, but the source of the vibration, if it only happens power off is likely to be the diff .

 

smoked pinion bearing and some bad subframe bushings will make it a noise conductor