I had this same problem on my radio, and fixed it with electronics cleaner from the hardware store (also available at Radio Shack). I'm a Creative Director at a TV station, and we see dirty "pots" (potentiometers) all the time on all kinds of equipment. You get a build-up of gunk (I think the technical term is "schmegg") and it causes that annoying static.

Here's what you do: Remove the radio from the dash. It's a good idea to remove it from the car completely, because dirty cleaning fluid is going to seep out of the radio as you're spraying it. Then remove the casing so you can get at the inside of the unit. Spray the volume control of the radio generously with cleaner while vigerously working the volume control. Try to spray the stuff into the crevases (after all, the problem is on the inside of this "switch").
If you have access to compessed air, it's not a bad idea to blow it out when you're done. If you don't have compressed air, let it dry out before you power the unit up again.

While you're at it, I would clean any/all other switches as a preventive measure.

Another word of advice, before you re-install everything, I would plug it in and check that the problem has been corrected. It may just need another good sqirt, and it's a lot easier to find out sooner than later.

tvpierce
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'92 Mercedes 190E (my daily driver), '93 Volvo Turbo Wagon (a family car w/flair), '53 Willys-Overland Pickup (my snow-plow truck/conversation piece)