Noise

Caroline hates loud noises.
Auditory defensiveness, I think the term is.
To the point where she is just starting to flush the toilet without covering her ears. Forget about using a hair or hand dryer around her. She just runs away.
I used to have to wait until she was in a sound sleep to vacuum when she was younger - she would freak out.
Sam, however, definitely does not have this aversion. The louder, the better (he believes in this theory when he throws a tantrum, too). He loves the vacuum.
To the point where it takes me twice as long to vacuum because he does things like this and I have to steer around him (and mind you, the vacuum is on):