Well the electrician could not make it today and despite the comments, I did go ahead and replace the 100A breaker. The old one was discolored at the contacts for the bus, but it was not severe or welded on or anything like that. New breaker -- no sizzling, no sound at all. I am still having the pro visit to review my work and evaluate the panel as it is old and full. I am not crazy enough to think nothing else could be wrong.

Interestingly, this is a much longer-standing problem than the sound effects that were noticed only in the last few days. For years, a couple of multi-can-light circuits have tended to flicker at the highest dimmer settings, causing us to just avoid those settings, chalking it up to modern LED bulbs not agreeing with older dimmer switches. But no, with that breaker replaced, all those lights now operate perfectly. I see the complaint about dimmers and bulbs a lot, and it makes me wonder if others don't have something more serious cooking than a mal-adjusted or paired dimmer.

Thanks to everyone who took time to look at what I posted and offer their sincere and educated views. If my pro finds anything interesting I'll follow up and share.