Twisted99

Just picked up a 92 SC400, completely stock, even has the "car phone" in the center console. Anyway the problem I'm having is a "thumping" sound from the radio. If I select CD, I get "err" on the display, have not tried the cassette, well, because I don't have one. On Am or FM I get this loud thump like the volume is being raised and lowered rapidly. Weird thing is it did it when I picked it up and then stopped on the ride home. Now it's back.
What causes this? Probably going to go aftermarket, but want to know if the factory amp (nak) is still good.