My tummy does this, too. I know the blood from your digestive system gets shunted into the pelvic organs, so it could just be the last few stomach or intestinal contractions before things quiet down and a good portion of the blood gets to where you need it.

And, it does seem to happen right at the beginning. He's going down on me, and I'm thinking, "Does he hear that? My tummy is growling."

Also, My Man usually has a strong urge to poop after we have sex. (TMI?) We don't play with his butt, so it isn't from stimulation. I don't know, stuff kicking back on, after the blood shunting? Just guessing.