Propane regulator has begun to whine quietly

My regulator was silent, until this last trip. Now it emits a very quiet whine or whistle noise. I fiddled with the tank valves -- no change (until the valve was completely closed, of course). It's not leaking, as far as I can tell.

I could just ignore it, but I'd like to restore it to silent operation. We boondock in some very quiet places, and it is faintly audible from inside our trailer.

What would cause the regulator to begin whining? Are there "consumer serviceable" parts within the regulator, or is it sealed to guard against dangerous amateur repair efforts?

Your advice will be greatly appreciated!