I think I've got it.
At low RPM's as I move slowly from a standstill to speed, I get that grunching sound/shudder for a moment or two until everything gets itself together. My belt and rollers are relatively new. I changed them just before the winter set it. If I accelerate quickly, no sound. Nothing unusual at speed either.
So I'm assuming the problem is with the clutch pads/bell engaging smoothly.
It's more of an annoyance than anything...still...
Before I take things apart, my search results tell me to blow out the dust, check the inside of the clutch bell for any unusual wear, and abrade it a bit with green scotchbrite pads. Anything else? Should I do anything to the clutch pads?
I've also come across some people saying if the bell is out of round, that could cause a similar problem. How common is that?