Depending on where he is -- I think he's on the East Coast of the US, where SW signal strengths are strong -- it could still be receiving SW on the whip, and he may have forgot to switch the BFO.
I just threw that suggestion out just in case, because I had a similar thing happen to me where I thought it was the radio and it wasn't.
Without an external antenna hooked up I'd check for RFI before returning the radio.
I think Tominbelfast has another SW radio -- if the same noise is on the other radio, it would probably be RFI.
If it isn't, you're correct, it may be something wrong with the World Receiver.
Either pull the batteries out and wait 15 minutes and let the microprocessor reset, and see if that changes it, or just take it to Radio Shack and replace it, they still carry the radios.