What could cause hammer-like sound at water service entry point?

[Theo N.]

I believe I hear a shuddering sound emanating from my 1" Uponor PEX water service line, puzzlingly during times (like at 4:00 AM) when we have no open valves in the house and in fact the house has rested for hours.

The underground line surfaces in the crawlspace of my 1-level ranch. It's then insulated, and travels in an approx. 5-foot run mounted to the timbers at (I think) two points. it enters the dwelling's garage, through a hole bored in the sill plate. From there it branches back into the house through various runs. I don't have an expansion tank or backflow device in the system.

The sound began in December 2017 right after I installed the 1" Uponor. There was no shuddering sound before that time, and no shuddering sounds in any other part of the house other than from right over this area of the crawlspace.

Can anybody shed any light on this shuddering sound? I have flushed the line a couple times since installation to no effect. It seems odd (to me, at least) that the city water service could stimulate quick pressure changes that would case the shuddering. But I don't know.

 

[Reach4]

That is weird. No flow during the sensation. Water softener is not regenerating. 1. No pressure reducing valve (PRV)? 2. How long does each event last?

I would buy a garden hose thread (GHT) pressure gauge, that has a lazy/tattletale hand. Just after a shuddering, check the hands. I am not saying that is going to detect your shuddering, but it is cheap. You can connect to the cold laundry line, the drain for the WH, or an outside faucet. The reason I said cold laundry water line is that it would see any pressure change less attenuated than the hot. Outside faucet would be best for that, but it is harder to access and could freeze.

3. Is there a nearby city water pumping station?

 

[Sponsor]

 

[Theo N.]

Thanks, following up on your questions:

1. No pressure-reducing valve.
2. Each event lasts about 1 second; an event consists of between 1 and 4 staccato sounds, and then that's it.
3. I called the city and they said I am on tank-fed gravity flow, but the helper said she would query the technicians and see if any pumps or reported anomalies exist close to me.