Well, Lets use the process of elimination...
Squelch. Makes some sense. Squelch is a soft sucking sound make when walking to trough something sticky or wet...like mud.
Pitter-Pater. What does that make you think of?
Maybe the tapping of feet or foot steps. So that wouldn't really make sense in the sentence would it?
Slurp. This is more for when someone is eating or something is being sucked up. So this doesn't fit.
Thwack. That sounds like someone hitting someone. Doesn't fit either.
Whoosh. The sound of something shooting or passing by fast. Doesn't fit either.
So what do you think the answer is now? :)