Need some advice:

At approximately 1800 RPM and with the engine under a load there is a hum that seems to come from the drive line. If the car is coasting the hum seems to go away but returns when accelerating or under load. I am thinking about taking the car in to the shop this week but thought someone may have had a similar experience.

The car is a 1984 300D Turbo with 155,000 miles. The transmission fluid was changed 15,000 miles ago and is at the exact correct level when checked. The differential fluid was changed last weekend (by me) and is at the correct level. I inspected the flex disk close to the differential and it appeared okay. Since the car is so close to the ground I could not see the center bearing. Could this be causing the hum?