I'll second the dirty volume pot (or other potentiometers in the signal path)... even if they make noise other than when they're swept.

Is your dad a smoker? We smokers find such insidious ways of getting crap deep inside electronics - I don't smoke in the house anymore.

The pots can be cleaned any number of ways - most here like the Caig products (I've never used them).

Bren R.