With the shower on, I hear a thumping noise behind the wall and the water pressure immediately drops. This happens every few days, so not every time I take a shower. It happens almost immediately after I pull up on the lever to transfer the water flow from the tub spigot to the shower head. Sometimes, if I turn the shower off, push down the switch to drain everything through the tub spigot, and wait for one minute, I can turn the shower back on and the pressure is back up. This doesn't always work, though.
The pressure, once dropped, will remain the same through both hot and cold temperatures. The only recent work done in the house was the replacement of the air conditioner unit and the replacement of my expansion tank above the water heater. The sprinklers are not usually on during the time I take a shower.
I have been unable to find any sort of leak or water stain anywhere. The noise, which I would describe as halfway between a clunk and a thump, with a dash of bump thrown in for good measure, sounds like it is coming from behind the shower, between the tub spigot and the shower head. My unit is a combination bath/shower, if that matters.
Any ideas or suggestions? Any other information I can offer? The last thing I feel like doing is plumbing work, but if there's a serious problem coming my way I'd like a heads-up and a chance to fix it before it makes a mess.