Hi

'91 Fourtrak 2.8 TDX.

I had a similar sound on two occasions, (1) After my truck was left standing for a month, re visit to OZ from the Uk, I was told by a tyre company this was due to a flat spot due to truck left standings , with low air pressure in tyres ( not sure about that),though it disapeared after a couple of days.

(2) 1mmediately after a emergency stop, locked up rear tyres and skidded to a stop, this was due to pulling on handbrake at same time as footbrake, thus locking up the rear tyres and wearing a flat on the tyres tread, this took a few weeks to dissapear. This sound was a soft thump,thump,thump increasing with road speed. Both rear tyres sounding in sycn with each rotation at first, then out of sync as time went on, due to due to distance travelled by each wheel.(due to bends/roundabouts).

Thought has just come to , once helped a lady whose car was making strange noises at the rear, on investigating it turned out that one set of rear wheel nuts were loose, this was after the garage had changed her tyre due to a flat.

Check for loose/worn bolts holding the prop shaft to the rear diff and gear box, (clunk and vibration), noticable when you apply or reduce power/ forward speed.

Wear to the prop shaft U/Js,(click, slight wear ,to a clunk and vibration badly worn))on take up of power/moving off from a stand still.

Has prop shaft been removed recently, if so it could have been refitted out of alignment,affecting balance,(vibration)

Tyre/wheel balance, are there signs that balance weights may have been fitted, that are now missing ??

Is any tyre tread missing, aggressive/chunky tyres can sometimes throw off a block of tread affecting the balance of the tyre ( remoulds/retreads)

Hope this helps .

Edward (ews)