The water in an upstsirs shower loses water pressure after it runs for a bit and at one point start to pulsate like its getting air in the line.  It does the same if you don't use the shower and just use the tub faucet.  This ia fairly recent, like last week it started.

Other shower in upstairs seems okay, they both use the same upstairs water heater.  No leaks I can see and cold water pipe valve into the heater is wide open.  It only started to do this recently.  I'm going to check the sink faucets next.  I don't know if there's any kind of pressure regulators in the house, knowing how quick these houses were put up the contractor probably left them out.  I don't know if our local codes call for them.

Upstairs water heater is about 8 years old.  Ideas?

I’m no plumber but maybe the cartridge is the issue.

If it uses a cartridge and you can find thenpart it should be pretty easy to diy replace it.

Thanks  my daughter said she'd hear a noise coming from the valve just before this problem started so that's the next step.  The video will come in handy.

No problemo

We had a similar issue (loss of pressure but no water hammer/vibration) and it was due to pieces of an old water heater dip tube making it into the shower valve and partially plugging it.

Is it only the hot side with the water pressure problem?

Yes, I'm in the process of removing the valve as I write this.

I replaced the cartridge.  There's a small diaphragm on the intake nozzles that came/tore off and was flopping around inside interrupting the flow of water.