I was able to determine the expansion tank was bad, I bought one and attempted to replace it, but it wad tight as he!! and I didn't want to damage anything so I had a plumber come out. He agreed it was bad, and said he usually replaces the exp. tank and prv at the same time, but we'll see if it was just the tank. So far so good, but that was only about an hour ago. I ran the sink and didnt hear it. He was able to hear the thump before he got started. I will give an update in a day or so if that was indeed the issue. Thanks for all the replies!