I didn't know that hammer arresters were code in some places. So far I have one on each of the lines for my washer. Since writing the original message I have placed one on the dishwasher, which was just as bad or worse. I may stop by the plumbing shop this weekend to get a few more to add to all three toilets. As for the pressure test, the arresters seem to take care of the bulk of the symptoms once installed. I'll still swap out the valve in the new toilet since I have a spare one. I know I have a slight leak in at least one copper line, which is burried in a wall behind kitchen cabinets. (Drips occasionally show up in crawlspace.)

I recently had a seal fail on the toilet fill valve in my master bath. I noticed a musty odor, kind of like how a crawl space smells, and started investigating. I found that the tank was slowly dripping, and because the floors aren't level (anywhere in my old house), the water was seeping under the sheet flooring--I have no idea how long, before I noticed the odor. I pulled up the flooring to allow the subfloor to dry, and replaced the fill valve. The flooring needed to be replaced anyway, but I'd been putting it off, so no big deal. But with the flooring gone, I started noticing a slightly damp spot in the subfloor, just at the front of the stool. I think that the slight swelling of the damp subfloor on the supply line side had unseated the toilet and broken the seal of the wax ring. I'm getting ready to put down new vinyl flooring, but the subfloor is not bad enough to replace (IMO), so one question is--would it be better to use another wax ring, or a SaniSeal, which might be a little more forgiving of a slight difference in floor level? I'd like to try the Fernco seal, but with the old wax on the toilet horn, I don't think it would work. Also, should I shim to level on the low side before installing the flooring? Shim and caulk after the toilet is seated, if I use the SaniSeal? Thank you. (X-posting to the Plumbing forum.)

You might try posting your question on a dedicated plumbing forum like Terry Love since you haven't gotten replies here.

Thanks for the advice, guys. I think I've narrowed the problem down to the fill valve. Even when it is in the upright position, the water continues to run into the tank. Don't know why it eventually stops, but it appears that I will have to have it replaced. This toilet is fitted in a fairly tight spot between the vanity and the tub and I'm in my 70's, so it is difficult for me to crawl around on the floor to get it done, but it is such a simple operation, I hate to spend $150-$200 to have a plumber do it for me. Wish me luck.