Depends. Sometimes a little moaning is cool. A little bit of bed squeak rhythm is cool. Sometimes the hard pounding smack sound does it. I've been on my way to an appointment and, while walking down the hallway, I can hear two people going at it (I wondered if it was another appointment, or a couple...probably the former
). As long as they're not too freakish sounding, and especially NOT a cry for help (idiot), they all tend to sound pretty good.