Note that some Volvo units use digital controls so the volume
is just really sending signals to a CPU to change the volume (spray
cleaner won't help with these).

If you have a digital control you can spin the knob 360 degress (or
more, it never stops). You can also spin it real fast and the volume
won't change or spin it with the radio off and the volume won't change
(perplexing children everywhere who like to get in the car and turn the
radio all the way up :) ).

I worked in an auto installation shop (as a teenager) and we used the
spray from Radio Shack all the time for noisy volume controls. I think
it was called TV Tuner Cleaner (or something similar). We would take off
the knob and spray some down the shaft then rotate the volume control a
few times. If that did not work then the radio was removed, top taken off
and the spray applied diectly to the pots.