Very annoying Nick and a real pain to find the cause-
another long shot for you but worth checking as I have just had a similar issue with my 240.
Check the three bolts that secure the rear control arm bracket, two on my car had worked loose giving an intermittent rattle/vibration.
You will get there
Good Luck-Bob
Link: ~
https://www.volvoforums.org.uk/showt...=282949&page=2