that noise (in my opinion) is the greatest detail in that scene. sollozzo has michaels (and the audiences) full attention right from the beginning of that scene. michael was so focused into sollozzo it was intimidating. after coming back from the restroom, he pauses. i think, he was somewhat questioning his choice of action and debating following through with it. then he sits, tries to keep his cool, but his mind was just swirling with emotions, thoughts, opinions, anything and everything was going on through michaels head at that moment. this was going to change his life forever and he knew it. i think that noise symbolizes all the "chaos" going on in his head.
robert