My 1999 with the stock speakers and cassette did the same thing. Turns out there was a loose ground in the steering column that was causing random noises, in groups of five. I had the loose ground fixed, then I ripped out the factory crap cassette and speakers and replaced them. I would highly suggest you do the same if you care about your sound. As for the problem, if it only happens in F.M. it may be you're trying to pull in a weak station (our stereos suck at this) If it happens all of the time, then I'd say it's some type of electrical interference, and I'd have the dash grounds checked.
