In article ,
Fred McKenzie wrote:

1. Microphone picking up sound of a fan.
2. Microphone magnetically coupling to power transformer or fan.
3. Bad shield connection on microphone cable.
4. CW Key plugged-in and keyed.
5. Monitor feature activated, providing audio feedback.
6. PL tone activated.

7. Decoupling/filter capacitor in V+ line to audio amplifier is bad,
allowing audio to couple back to earlier stage that shares the same V+
source. Oscillation is sometimes in the form of "motorboating", a low
frequency relaxation oscillation.

Fred
K4DII