Nasty Knocking noise @ over 70mph

I have a Ford escort Finess, 1.6, 1998, zetec engine.
At over 70mph there is a nasty banging noise coming from the rear of the car like someone is sitting in the boot bashing around with a hammer.
The wheel bearings have been checked and are ok, I have had a new exhaust fitted which is not hitting anything, I have had several people look at this but no-one can come up with the reason..any ideas??:banghead:

Is there a welt in the tire? Maybe tire out of balance?

Thanks, unfortunately not, have had all tyres checked..
Someone suggested a loose spoiler..could this make that much noise?

Yeah. It could be a spoiler that's loose if you have one. Could be a loose strut, but it would do it all the time and more on a rough road. You're just funning us about a hammering sound in the boot. Aren't you? Even if you had a new exhaust fitted it could still be going up into the body if the mounting is escessively loose. Get under there and push up on the muffler to see if it might be driven into the body from wind resistance.