Simple groaning, moaning of pleasure should do the trick. Yet, my dormitory neighbors don't seem to know it. At least once in a week I'd hear:
"AGHHHHHHHHHHHH"(male) and "KYAAAAAAAAAAAAA" along with "FUEEEEAAAAHIEEKKKKKITSSOHUGE" (female)
I mean, hell, I guess it's good when whoever's dick is huge and all, but I wouldn't need to know. I can barely talk to my neighbors these days.