The vibration is coming from under the car. Very noticeable between 65-85mph, so local driving ok, just motorways are a problem. It starts to build from 65 when accelerating but gets worse when ease back to constant speed. It is a rapid uneven thudding, hard to describe. No noise, just the vibration.

Can feel it through steering wheel and gear lever though there is no movement in the steering wheel as you get when wheels are unbalanced. You can "feel" it through the front seats and any passenger in front is now picking up on it. Has gradually got worse over the last month or so.

Had wheels balanced 4 times in last 8 months, wheels have been changed round and now propshaft has been given all clear as we thought it might be a bearing. Garage can't find anything so just wants to leave it to see if it gets even worse.

A friend has suggested that it maybe the flywheel which is a major job as I understand that the garage will have to drop out the gearbox before they can even see if it is the flywheel.

Car is 2001 with just over 50k on it. My third 5 series, never had this problem before and get the sense that the garage is getting fed up of it too!

Any other ideas?