Buzzing noise out of the speaker

Hi there,

I recently bought a Classic Gameboy from a "flea-market" and I cleaned it and put new batteries in and it works fine so far. There is only one drawback - there is a really annoying buzzing noise coming out of the speaker. This is not changed by the volume regulation and when you put in earphones the buzzing noise is coming over them as well and as loud as out of the speaker before. Any ideas how to fix it?