Going through a set of double doors, a person holds open the first door for the person coming behind. If the recipient of the open door is polite, they're almost sure to respond with "thank you" or "thanks."
Next, quite predictably, the helpful person in front holds the second door open. The follower isn't quite sure what to say. He just said "thank you" five seconds ago, and saying it again just seems like politeness overkill. However, to abstain from saying thank you could be interpreted as rudeness. What to do? More often than not (pay attention - you'll see it's true), the recipient of door opening #2 will say, in a voice just above a whisper, "Thanks." In fact, the "thank" is almost inaudible, and all that remains is a "ssss" sound.
In fact, "ssss" is the universal sound of thanks for the opening of the second of double doors. At malls, hospitals, office buildings - indeed wherever there are double doors and people holding them open - you'll hear the "ssss" of gratitude. Watch and sssseee.