WJcandee
Wise One
I'm kind of afraid to do this. I don't want to make things worse. But I will try it. Sounds like I should
You're afraid to turn off the little valve at the wall that feeds the little hose or pipe to your toilet? I would have thought that would be one of the first things you would do. It's there to be used. Use it.
By the way, if it is not all the way open or all the way closed, that could be part of your problem. Try closing it all the way, flushing the toilet, and seeing if you still get the thud. Then open it all the way and see if that doesn't solve your problem. It probably won't, but it's worth a try.
Anyway, report back to us whether it thuds with the water off, please. That dramatically narrows our inquiry.