Clunking noise from shower faucet

hi

My shower has a Symmons temptrol shower faucet. within the past couple of days, I've noticed a loud clunking noise when I either turn the water on or off. if I'm moving the handle slowly enough, the clunking is still there, but now takes on a stuttering quality. other than the noise, there are no problems....water flow is still fine, and i'm still getting hot and cold water.

Does anyone have any ideas what could be causing this? I was thinking it could either be the washers, or a loose pipe. The washers I wouldn't mind replacing, but I'm hesitant to just go opening holes in the drywall to find the pipe.

thanks in advance