That problem is common with older electronics and is caused by dust in the volume-control pot.
Electrical contact cleaner spray usually does the trick. If you don't want to try this, a professional cleaning at a local electronics repair shop shouldn't cost much.
While you are cleaning the volume control, be sure to clean any other rotary dials such as the tuner and the source selector or balance controls if it is a receiver. Dust in any of these can cause problems.