Vauxhall Vectra V6 estate knocks at the rear?

Vauxhall Vectra,V6 estate 99'. Knocks and rumbles at the rear continuosly.Over 65mph bad vibrations felt throughout the car and on steering wheel.I've been told it's probably drop links but i've been right through it all underneath,on ramps and stands,checked all the moving parts/bushes etc with a lever bar and can't find anything amiss.Any ideas? Cheers.

Trumpy
March 2009