Hello, all Volvo enthusiasts!

I need an advice or idea about the following... My car is 240 DL wagon 1987 with 239K mile already. Starting from some time (approximately a year or two ago) there appeared a howling noise coming from rear. The nature of this noise is strange. On low speeds it is not heard at all, but it starts gradually to increase its volume as the speed approaches 60 mph. On this speed it has the highest intensity. But then, as the speed passes the 60-mph boundary, the noise starts gradually to decrease. And more strange is that it happens only when I press accelerator pedal. If I release the pedal, the noise disappears... People say it could be the center support bearing. I replaced it a month ago. The noise is still there...
What could it be? Dieing main fuel pump? Trailing arm bushings worn? They haven’t been replaced since the birthday of this car. Rear differential? But I changed oil a month ago. Rear wheel bearings? U-joints? Driveshaft out of balance?
Actually, this noise doesn’t have any impact to the car’s drivability, but it’s very boring and uncomfortable...
Any ideas?

Thanks in advance.

Dmitriy.