Loud vibration coming from pipe, Pretty sure it's NOT water hammer
This is the first time I've posted on this site. I know there are many threads about water hammer but I'm pretty sure that's not my situation, I could easily be wrong though. I live in a two story ranch house with the basement unfinished and the top floor finished. About a week ago my wife heard a loud noise in our basement that sounded like it came from our copper plumbing pipes, she said it only lasted a few seconds and then stopped, she said it happened several minutes later and stopped again. I was asleep so I didn't notice it. The next night before I went to bed I heard the noise so I ran downstairs to the main line that comes in the house from the street and sure enough it was vibrating so fast and loudly it was almost as loud as an air compressor. I put my hand on the pipe and could feel it vibrating. It only lasted a few seconds and then went away. I have lived in this house for 35 years and have never heard the pipes make a noise like this. The next day I went and bought a water pressure gauge and put it on the spigot outside closest to the PRV, it read 140 PSI. I knew that was too high considering the current PRV was set at 50 PSI from the factory. So I went out and bought a new Watts brand PRV - same model as the old one but lead free now, and replaced it along with my main line shut off valve. I had the old style shut off valve that you have to turn a hundred times to turn off or on so I replaced it with a quarter turn ball valve. After I was finished I turned the main valve back on and let water run out of the tub, faucets, etc so all the air was out and then I checked the pressure again and it was at 50 PSI so I figured that would fix the problem. Unfortunately it didn't, I've heard the same noise every day at least once since then and I heard it earlier tonight and it lasted longer and was louder than I've ever heard it.
A little about the house:
All the supply pipes in my house are copper.
The main line comes in from the meter up close to the road and underground and under my driveway into my cinder block wall and up the inside of the wall to the plumbing pipes that are attached to the bottom of the floor joists.
About a month ago or so I got a letter from our county water system stating they would be replacing the meters in a few months and they would be installing a back flow preventer on the meter. The letter went on to say that I would need to install an expansion tank to my hot water heater if I didn't have one (which I don't right now). I thought the county messing with the water line might have been the problem but they haven't even touched my meter yet and I haven't seen the working in the neighborhood either.
The noise DOESN'T happen while a faucet is on or the water is running or someone has just flushed the toilet, it will happen out of the blue when no water is running in the house.
If anyone has any suggestions or solutions I'd be extremely grateful because the noise/vibration will drive me nuts out of worrying and I don't want to mess up my appliances or pipes.