Silent Sex???

I live in a share house and one of the guys I live with has a girlfriend who from time to time spends the night. On more than one occassion it's struck me as odd that I've never even heard a hint of them having sex. Our rooms are joined by a common wall and while our wardrobes are both on that wall I hold no delussions that he has not heard me through it from time to time. I can hear his tv through the wall and even him sneezing. It just creeps me out that there is never ANY noise coming from that room! I was first alarmed when she spent an entire weekend at our place, they spent the whole time in his room (which is all they ever do), the house was silent and still NOT A FUCKING PEEP!!!! So now I wonder have you ever had sex with someone who didn't make a peep? I have and let me tell you it was WEIRD! Not a moan, not a gasp, NOTHING!