Hi Martin here,
There is nothing more annoying than a vibration or a rattle, drives me mad too. The drive shafts are the problem here, the list of checks is as follows: worn drive shaft joints, worn drive shaft splines, loose bolts or retaining nut in the center of the wheel. a worn wheel hub. This will be a process of elimination, to be checked through one by one. If the bolts and nut seem fine the shafts will have to be removed to check the splines and the hubs. Before removing the drive shaft jack the car up, place it on stands, let it run in drive ans listen for any vibration noises. If there are no vibration noises after this test switch off the car, grasp the wheels by hand on the top and lower end and pull on them, with a rocking motion, to feel for any play which might indicate a worn hub or wheel bearing. These are the only suggestion which come to mind as you say the mechanic has already checked the mountings. Should you require further assistance feel free to ask.