Specifically, my Icom ID-4100a.
What's interesting, is that this happens in MR and VFO modes... but NOT when using the built-in WX stations. For example listening to WX-01 (162.550 MHz), no problem. Listening to 162.550 via VFO and I get the problem.
I first noticed it during a local repeater QSO and thought it was the other station experiencing an intermittent fault... and then noticed it with another station later that day, so it had to be me. Opening up the squelch let me catch it in the act, and it seemed so regular that I thought to time it: every 5.5 seconds on the nose.
If you have any thoughts or suggestions for further diagnosis and remediation, I'd love to hear it! My instinct is to first try a Partial Reset followed by an All Reset before cracking open the case on the workbench.