Check valve question

I moved into an 11 year old townhouse last year. Replaced HW, expansion tank, and PRV valve (house pressure was 100 psi).

I occasionally hear a short – quick tap noise (sort of like a medium pitch chirp) in the wall from in one of the upstairs bedrooms. Looking in the attic above the location of the noise the only plumbing I can see is a PVC pipe which exits out the roof. The adjoining townhouse is a single story unit - i.e. there is no structure adjacent the bedroom in question.

I’ve heard that PVC pipe can expand and contract due to temperature changes – I’m thinking that may be what it is (I’m not there during the day and it only hear it late at night). It’s sporadic (happens once or twice in an hour then stops) and doesn’t appear to be related to water hammer (does not occur when washer, ice maker, etc valves open/close). I usually hear it once before I go to bed around 11pm. I live in Central PA.

What I wonder is if it is in the copper pipes elsewhere – could a faulty double check valve be the culprit? The check valve is original (as far as I can tell) and is a Wilkins model 700. The check valve is located after the meter which is located after the PRV valve. What are some of the signs of a faulty double check valve?

Thanks

PVC drain pipes change size a fair amount with temperature changes. If you take a hot shower before bed, you may be hearing it returning to it's shorter length after expanding from the hot drain water.

Copper supply pipes will change length, too, after running hot water through. If they are run through a tight hole, they could make noises when either heated or cooled.

PRV's tend to make whistling or groaning noises when failing. I could also make a hammering noise.