That was me being stupid.
I'd originally set the cass. deck up on this cabinet in the music room, without thinking. The opposite end of that cabinet has the household WiFi hub on it.
Without thinking or realizing, I attempted a transfer, and, I came up with this weird pulsing sound. My mind tried to turn it into a bad capacitor in the deck. So I opened it up, and looked for overt signs of bad caps; leaking spots, split cans, etc.. I found nothing.
I tried a second transfer, and, came up with the same noise again. WTF?
Then I realized my mistake, and, moved the deck over to another outlet, and tried again; no noise.
I took the pic before I put the lid back on.