Can be quite quiet.
I had one sneak up on me once and I never heard it coming. It came from behind a 500' ridge, on the down(fairly brisk)wind side, flying way below ridge height. As it got nearly on top of me, it pulled up very steeply, popping up from out of view with a sudden rush of noise and displaying the dark underside of the body, which blotted out the sky. It frightened the life out of me, then changed heading 180 degrees and flew off to the south until it disappeared over the horizon. I rather suspect that the pilot did this on purpose.