rear suspension knocking

hello all
I can hear a knock coming from the rear axle of my 95 4.6 hse,it comes when both rear wheels go over a bump and drop down again,it's more like a metal touching metal sound not a bang.i have replaced the rear shocks a weak ago and it didn't cure it.my mecanic checked the rear bushes and the anti roll bar bushes with a lever and no play was noticable.i have checked the exhaust mufflers and pipes and mountings all good.this knock is more heard if i drive over bumps on the bump stops.when the windows r rolled up u can't hear anything,it is heard when windows down.wht could tht be i am getting crazy with tht knock already.and by the way the top shocks bolts r torked as required.superman power.
Please any help.

derek any help?

Sorry Jorjio, you appear to have done all that I'd have suggested. Can you 'feel' anything through the seat? Your exhaust is the favourite for hearing bangs but not feeling them but if that's all been checked, I don't know I'm afraid. The only other thing that comes to mind is the rear prop shaft UJs. They may be at such an angle when the suspension is compressed that, if they're worn, you may get a rubbing sound briefly until the car is level again and the propshaft is more aligned. Make sense? If I think of anything else I'll post it.
Good luck.