AM Band buzz

I recently bought a SR3 from a seller on e-bay, and it has developed an annoying buzz. This happens only when using the radio on AM. When listening to the radio, everything is perfectly clear, but every 20 seconds (measured with a stopwatch!), it gives a buzz lasting for about a half-second.

Do you have any idea what could be causing this? Or, where you might direct me?

Thanks for any info you may be able to give.

Grace, Peace, and most of all, Love.

Jim Harvey