I have this happen too! 42mm Stainless Steel.
I thought at first it was just the strap or strap connectors clicking when raising my wrist to view the screen, but today I isolated it to the speaker on the device. It doesn't happen every time, but enough that it's a mildly annoying quirk.
Also, if I take my watch into a very quiet room, hold the speaker slits to my ear, I can hear extremely faint white noise coming from it when the screen is lit. So for some reason when the screen turns on on the Apple Watch, the speaker wakes up too, and when the speaker wakes up, it sometimes emits an clearly audible popping noise.
Also of note, the speaker functions perfectly fine when on. Notification sounds and calls sound crisp and clear, just for some reason it pops when turning on the circuitry that controls the speaker.
My device has never touched water.