If the check valve is shot (and unloader is OK), it will continue to
hiss (constantly) until the compressor kicks back on. This is common.
The compressor will try to cycle frequently to restore the pressure.
Everything will seem fine while the compressor is running. Once it
reaches pressure and shuts off (actuating the unloader), it will hiss
constantly again, etcetera.

Yup. I went out and played with my C/H today. If it shuts off on it's own
(switch made) it hisses air. If I turn it off early from the disconnect it
doesn't "unload" and it grunts when it restarts. (I didn't try that too many
times).
I vote for the unloader being bad too. Sorry about the check valve FUD.
(Unless machine this predates unloaders).