Rear end noise

[Snap-on]

Hey peoples,
Having a bit of dramas with my VT wagon. While driving on semi smooth to smooth roads, you can hear a "whomping/romping" noise. Eg, "womp, womp, womp" It will increase with speed, and in any gear and neutral. It sounds as if i have square tyres or something.
I originally had 18's so I changed to pursuits with new tyes, but still the exact same noise. I have had the diff rebuilt done in the last 6 months, all new suspension bushes, 4pt rear camber kit etc. I have had recent wheel alignment, with a rear camber setting of 1.75deg - on SL springs.
I have taken it to 2 mechanics now, and both can't work out waht the problem is.
I am wondering if it is rear wheel bearings, but when the car is on the hoist, their is no noise and the wheel spins freely and quietly.
The noise does not change while going around corners or anything like that. I am really stuck on this one, because I repair most things on all my cars and generally have a good idea what is going on.
Anyone had this happen before?? Any ideas??
Thanks in advance.

[ToMMMaY]

whats the bet your driving over reflectors on the road hahaha

anyways tho its hard to say without hearing it yourself put someone in the back of your boot and go for a spin and find out what side the noise is coming from then start there lol

[Tree cutter]

Could be a uni joint or centre bearing in your tailshaft.

[azzaVT]

Or a warped/bent or unbalanced tail-shaft... having said that... check engine and trans' mounts, although they're more likely to cause clunking on accel-decel', but if the alignment was completely screwed???