This sounds like a noisy/dirty volume control. The volume

control uses a flat metal plate that sweeps across a wire

coil (simple explanation of a wiper type potentiometer).

You could have dirt between the coil and sweep arm.

You might be able to get some tv tuner cleaner from Radio

Shack and spray it into the volume control (remove the volume

knob) then turn it back and forth a few times. This

could work if it is dirty. But, a lot of potentiometer are

sealed and the problem could be that it has just worn out

and needs to be replaced. You might also have to remove

the radio (easy) and open it up to spray in the cleaner.

Removal instructions are here:

http://volvospeed.com/Repair/sc816.htm

Note that newer Volvo radios use a digital volume control

so this won't work on newer models (my 816 is like this, not

sure about others). A digital control knob can be rotated

either way continuosly (it never hits an end point) since

all it does is send up or down signals to a CPU. If your knob

can be rotated continously, its a digital unit.