Frost free hose bib groans when not wide open

My frost free hose bib outside that has worked fine for a handful of years (its not that old) now groans when you turn it on.
If the hose bib is running wide open, the noise does not happen. If you turn it on just a little bit, half way, you have a constant groan.
It does not stop until you turn the hose bib to wide open.
It reverbs through the whole house.
No other fixture does this (inside or outside). The rest of the house is fine.
This hose bib does not leak in any way or drip. It seems fine except for the noise.
There is a shutoff valve on this hose bib so i can turn it off and fiddle with it.
That shut off valve is a ball valve and its open all the way during the summer months when I use this faucet.
I did take it apart recently and pulled the long stem out, and at the end of that stem was the screw and washer.
That seemed loose so I tightened it. Didn't help. (I don't know how loose or tight the washer and screw should be by the way).
I know there is another mechanism up at the front of the stem (just behind the handle).
I have not played with that.
Any idea what the problem might be?
Since it doesn't happen if the fixture is wide open I wondered if there is a washer or something in there rattling but I'm guessing.


Thanks.

Maybe, try replacing the washer, and screw it nice & tight.

It is probably the gasket in the "anti backflow" device. It is vibrating like a reed when there is not enough pressure to hold it tight against its seat. But, it could also be a worn faucet washer doing the same thing.