Re: Thumpin pipes
It's possible that third faucet could be to a spray hose somewhere, but then there should also be a visible cap either at the faucet or near the sink in the wall. That cap will likely be a standard garden hose thread, hard to say with such old devices, particularly if they've been abandoned. If this is the source of the hum/hammer, then you can test this theory by giving that third handle a twist and see if the sound changes or goes away. If this happens, then it's time to replace the seat washer, which is either very warn, disintegrating, or loose.
That third valve could also be a "fixed" temperature valve. The hot/cold would be set to the desired temp, then the third valve would turn the water on/off without changing the temp. Makes for easy one handed control.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!