With the nasal lubricant (they actually sell it - it's called Ayr), I hate the nasal pillows a lot less. But yes, there is the hissing noise. It amuses me, though. Without the hiss, my cat wouldn't perch on my chest to bat at whatever's blowing air at her.

It's less cute when she grabs the rubber attachment part of the hose and tries to run off with it.