Sometime last year, Sept/Oct timeframe, we starting to hear knocking in our plumbing. It seemed to be located between the main and upstairs floors in the area of the upstairs washroom and main bedroom. It was intermittent and stopped happening before we could really troubleshoot it.
About 3 weeks ago (late April) it started again and now is happening intermittently but regularly.
We have 2 sinks in an upstairs washroom. Turning on the hot water gradually in either washroom sink causes the noise frequency to increase and then, eventually, stop. Turning off the shutoffs (in any combination) does not change the noise. I cannot feel much in the way of vibrations in this plumbing. Additionally, the holes for the piping allows me to physically move and hold the shut offs (maybe 1/2") but that does not stop the knocking.
Turning on the water in these sinks can stop it for a brief while.
I have access to the tub/shower plumbing (although there's no shut off) and again there is very little vibration there. I'm uncertain of the plumbing between them.
Can a water hammer just suddenly happen and, if that's what this is, what would be some suggestions on the number and locations to install arrestors?
If it's not a water hammer, what else could this be?
FWIW, I replaced - and moved from one location to another in the basement - my water heater last April. I used pex instead of copper. edit I can feel vibration in the HW pex or copper connected to the heater but holding or moving the plumbing doesn't stop the sound.
We have not had any other plumbing work done.