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. 
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As an update, I received the package back from the buyer, and as expected work perfectly fine. I was confident about it all along it may be something within the buyers other components that's causing his issue. I will have to refund the payment but the question is am I within my rights to ask for the fees I've paid to Audiogon and PayPal because there was no issues with the amp and preamp? Its less than $100. Didn't want to get a negative feedback for no fault of mine. 
The truth always works for me.. Ask the guy if he thinks it's fair?
Did YOU offer a no question ask, money back deal? 

Fair questions for fair people.  Feedback works both ways.
Does for me anyways.. Some folks you can't do business with.

Come on you broke the key off in the lock, how is that my fault?...
Ok yea, I've hear it all.. Oh and it was a piece of sh%t to boot.. LOL

Regards