In case you have seen the movie you may have noticed that there are no such noises in this scene at all, on the contrary, we hear pleasant music while a particularly long and boring make-out takes place. The problem for me was to make it clear what is happening on the screen without showing it.
Therefore, I had to replace the music for the silly noises – which frankly just illustrates the ridiculous love action: Brad Pitt, rolling his eyes like an epileptic mule and taking three times longer to get out of his shirt than for the real act. Poor Susan. If that was her best experience, she must have had a very sad love life.