I've been troubleshooting an issue on my Dad's '90 740 Turbo wagon - the car has developed a low end rumble and high end vibration. The low end rumble happens when the car is in gear (D) and idling - if you pull into N, the rumbling stops.

The vibration occurs when accelerating and once the engine hits 2500-3000RPM and gets worse as the RPMs climb (also happens when revving in N, although not as prominent). A local mechanic suggested a worn mount, so we just replaced both engine mounts today and the rumbling is worse (likely due to stiffer motor mounts transferring the vibration into the frame). The vibration is not horrible, it just sounds like something is out of balance. My initial thought was perhaps an issue with the turbo? The car's performance seems to be mildly affected - not very noticeable, but I can feel that it is not running as it should.

Any thoughts on what could cause this?

Thanks,
Phil