Scares Rico, even now

When I went back to the hospital to see my old roommate Ken, who'd been readmitted, this sound was going in his room.
I asked "What the fuck is that noise?"
"Just the ventilator," they said.
It scared the shit out of me (and still does; I can only stand to listen to a few seconds of it at a time).
Amazingly (well, not really; it was running day and night for seven weeks), it's the only thing I truly remember from my time in the hospital.