Well, I solved the problem. It was feedback due to close proximity of
the transmitter and receiver, just as Chuck, Tio and Kim suggested.

Tio recommended an on-the-air check, and that’s what I did. I erected
a makeshift 80-meter antenna in the backyard--good practice for Field
Day--and called CQ. A station in New York replied. He reported that
my audio was fine, with no hum modulation.

What a relief! Thanks again for all the suggestions.

-Dave Drumheller, K3WQ