As soon as you said hissing I thought of Hannibal Lecter and OH MY GOD that would freak me the hell out if it was Anthony HopkinsReally depends on which Invisible Man you're doing (and like you said, probably not the best idea to go there).
With Drac, it could be him making sure you're ripe to become a vampire. Imagine hearing his hissing in your ear as he's looking for a new bite.