Bad vibes

[Nick Jones]

1996 4.2 QS with 4 speed 'box. 138k alleged - probably FAR more.

Have a problem with a rumbling vibration which appears about 60-65 mph, peaks about 75 before seeming to lessen (a bit) at higher speeds.

Not felt through the steering wheel and does not seem to alter much power on or off.

Wheels have been swapped front to rear with absolutely no change. I've not so far tried getting them balanced - tyres are crap (Triangle make, apparently a bit less smooth than square tyres acc. to internet!), and 40 section rather than 45, but plenty of tread and no obvious damage. Couple of the wheels (standard sport ones) show signs of fairly heavy damage in the past.

I accept that the wheels/tyres may wheel be responsible but would have expected to pick up some sign through the steering were this the case and I'm not, even though the dodgiest looking wheel is now on the front.

There is also a wheel bearing type noise, but I'm damned if I can tell which corner it's coming from - this is most noticeable at low speeds (<40) when slowing down on smooth surfaces. Possibly due to the tyres which are noted for being noisy.

Further possibly relevant points:

When warm there is a fairly heavy clonk when going from D to R which is also present though less pronounced when going from R to D. I've seen suggestion that this in combination with vibration might be the propshaft centre bearing of CV joint.

No obvious problems with driveshaft CVs

Torque converter lockup seems a bit random.

Suggestions welcome. I know I should invest in a set of better tyres the right size.......

Thanks

Nick