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

I was going with the tranny mounts also, you changed the motor mounts but not the tranny... My 89 764ti has the same prob, have changed the ctr suppt bearing/bushing, have to swap the diffy as that has some clunks and will be swapping the tranny mount shortly, this is the only 7/9 car that has not been well vibration insulated, should change after the mount swap. tom 27940aholic

I have nearly the same issue, all mounts are new, exhaust new w/hangers not touching even in drive w/ brakes on. Alt mounts new. On mine it feels like a misfire or vac leak at idle. at 1700 it is the worst and on take off it makes the car feel like a piece of junk. The performance is down and so is the mileage. My tranny has unknown mileage but the engine has approx 170,000 with a overhauled head approx 16,000 mileas ago. I am thinking I have a headgasket leak between two cylinders or a bad transmission pump or other bearing in the trans. There is a loud vibration and the car sounds like it has a very loud fan under the hood. ( clutch and fan blades also new).
I am going to do a leak down test and a compression test over the weekend and I'll report back. I have spent a lot of money and time on this car and this problem which is getting worse makes me want to buy something else. My last resort is to swap the auto tranny to a different one, I have one with 131,00o miles on it and I drove th car it came from it was good before I pulled it. I am just sick of working on broken things.

car is a 1991 780 w/ B230ft and aw-71 trans

I just noticed the other day the same thing on my 760 turbo SW 1990, with a 561 ECU and year old turbo, head (530 without sodium filled valves), motor and tranny mounts- do you think it might be the oil? Ignition timing? I started using Lucas on the last oil change, and it did quite down the valve noise, but is it possible that this is the cause for this vibration at idle, when at the light?
It is a mystery for now.

You could also have a look at the exhaust system and hangers. My vibrations improved after replacing the previously-cobbled-together exhaust in the car and re-using the hangers. The whole thing before was sort of binding and under tension, now it sort of "hangs" there freely.

Well since you say it gets worse in D, my guess is the transmission mounts.