Listen to what it sounds like when you have your head -right- beside your microwave when it's running... Your cats ears are so good that the power running through your house wiring sounds like that to them all the time...

I've seen things on the Discovery Channel that suggest that your cats hearing is better than your dogs... and their noses are pretty much equally good as well...

In the Big Norh American Black-Out, couple of years ago, my cats were visibly freaked out... by the sudden quiet!

LOL