Mystery frying sound

I'm on a Raspberry Pi 3B, using HDMI sound fed to a Dell monitor about
10 years old.  No internal speaker in it but I use an analog out jack
it has and drive a pair of amplified computer speakers.  It's getting
worse lately.

If I unplug the analog input to the powered speakers it goes away.
If I turn down HDMI sound in AlsaMixer it goes away.

I can make a recording in Audacity of tone, silence, tone, silence,
then play it and I near the frying on top of all like it's mixed in.

I have no other (like USB) sound card plugged in.  Where can it be coming from?