Intermittently, my brake pedal will drop slightly, and there will be a loud clunking noise, metal on metal sounding. Normally happens at slow speeds, weather conditions don't seem to be a factor.
sounds like something to do with the brake hydraulics. possibly brake booster?
Need to state more information like HOW & WHEN things happen... What CAR? etc.
The noise is very odd, but MIGHT be attributable to sticking brake components that suddenly come loose. Possible that some other kind of electronic control on your brakes is acting up as well, whether in reaction to sticking calipers or not. (Traction Control, Electronic Stability, etc.). Not sure if your car has these slightly more techy features.
Unless you work on cars, take it to a mechanic and get it diagnosed and fixed.
Brakes are safety critical so you dont want to leave it alone or do shoddy work.
If you want, u can get it diagnosed and come back here to help you not get swindled.
How hard are you braking when you experience this?
Do you know what the feeling of ABS turning on feels like? The pedal moves a little back and forth and you will feel the movement when braking, sound similar?
I am pretty sure there is some procedure to test the ABS. If anything, you most likely can at least do a resistance check on the electronic components, which takes like 10 minutes and no $$$ at all. Have someone do some sort of REAL diagnostics before simply replacing parts.