I'll pick up the valve next time in town. In the meantime, I scaled the valve all the way back to barely open. The reason being, something occurred to me.
A couple of months ago I had work done in the downstairs bathroom, up in the ceiling. At the time the plumber asked to change the supply line to pex; he was going to have to remove it to do his work and figured might as well change it rather than feed the old one back up.
Anyhow, while reading about this, knowing it'd be a few days before I went to town, something occurred to me. When I changed the toilet I recalled having to barely turn it to close it and thought "hmm, he didn't open the valve all the way"
Then another thing occurred to me. Before the toilet change, a common thing would be, when bathing my kids, they'd pee while the tub was running and I'd tell them not to flush but they would. Used to be it took forever for the flow to stop and the tub water to get back to lukewarm. Since I put in the new toilet and opened the valve full tilt, the toilet filled up really quick. And that was the same time the hammer appeared. So I scaled it back to barely open, the toilet is taking long to fill again like before, and the bagb is gone.
So looks like I'm ok until I get into town. Thanks all !!