1987 Volvo 740, 4-speed manual with electric overdrive, 4-cyl, 232,000
miles

I bought the car when it had 139,000 miles. There has been a vibration
in it since I got it.

The vibration occurs starting about 50 MPH and almost smooths out at
around 70 MPH. It's worst at 55 MPH. The vibration is fairly mild but
seems to get worse when trying to accelerate up hill in 4th or overdrive.
The frequency is a little faster than that of an unbalanced wheel. The
frequency varies with the speed of the car, and not the gear it's in nor
the engine RPM's.

I have had the tire balance checked, the tires and rims checked for
out-of-roundness, the U-joints checked on the drive shaft, the front
struts replaced (because they were worn out) and the vibration is still
there. I have had my mechanic, whom is very good, try to find it, but
he couldn't.

I have tried to balance the drive shaft by putting a hose clamp on it
in various positions. While this actually created a slightly different
vibration, the frequency appeared to be the same. The only items left
that I can think are the drive shaft is out of balance, there is
something wrong with the overdrive unit or there is a broken motor mount
or transmission mount.

Any assistance would be appreciated.