Amp makes clapping noise

I had a Peachtree Preamp and Amp that worked perfectly well and recently sold them here. The buyer hears a clapping noise when connecting the amp to his speakers. The preamp or other sources are not connected. I did a FaceTime call and it does happen. Any ideas what the issue could be? He is connecting them to Snell speakers which he says works with his earlier integrated amp. Any input is appreciated.