I've never heard a hiss, but other things clue me in. My rear flat today felt like thud thud thud or kathunk kathunk kathunk. I usually notice front flats by the change in sound and by the feel of the bike and/or loss of handling. I don't typically get the thud thud thud effect on the front.

When I took my new bike out on her maiden voyage a couple weeks ago, I didn't notice my front flat until it was really flat. I was attributing the strange handling to the new bike. Ummm, duh...it took nearly crashing on a corner to realize what was going on!