not sure why I never asked before, but my rear out door faucet makes a constant loud noise when running that's best heard in our bathroom. no sound when shut off. no obvious leaks (no spinning at the meter...)

any ideas?

What type of noise?

Is the outdoor faucet directly outside the bathroom? If so they probably tied into the tub/faucet/toilet cold line.

Does it sound like rattling from a loose pipe?

If you have a backflow preventer on the faucet it will cause a noise I can't really describe.

they probably did tie it into the cold water supply from the bathroom. Do you have anything attached to the faucet like a y splitter or a automatic timer? Yes, it also could be the backflow preventer

If it's like mine (kind of a screeching sound while you are opening the faucet), it's probably cavitation from the gate in your faucet in conjunction with the pressure behind it.

Open the faucet up all the way, or even just give it some time at a partial open, and the noise should go away.

there is a backflow preventer on it.

its not directly outside the bathroom, out bedroom is in between (faucet is on the bedroom wall)

hard to describe the sound... more if a rumble then a rattle. almost like cavitation in the pipes... sounds kinda like a brass instrument being played badly...

every outlet should have a "hammer arrester" (essentially a Tee-about 6" long that should never fill with water if it is vertical and upright to pipe near outlet. This allows any "surges" to relieve in the air in the little pipe. If the pipe somehow gets filled with water it will not work.
To check and fix, turn off your water main and open all faucets and valves downsteam from cutoff and allow an hour or so for the entire system to drain out of your house - this will clear all the "hammer arresters"... Then, turn back on water at main and gradually close all outlets from the bottom up. This will refil your pipes and should leave the "hammer arresters" clear until it needs to be done again.
Just a thought.