I hadn't driven my S for over a week, and it sat parked. It rained all week, and then yesterday, I finally got back in. When I first started to take off, there was a knock (louder than that clicking from the rear many have heard... i have that, too and it's different). It was also accompanied with a resistance to get going. Almost like my brakes had gotten stuck clamped down. It just clicked once, and it only "held" me there for a fraction of a second, before I was accelerating. It hasn't happened since. My parking brake was engaged all week (though not when I tried to take off). Is there a chance it had stuck a little, and then it released after a half second?

 

That's probably what did happen. It's happened to me before. No big deal, as far as I know.

 

Yep me too left car at airport for 6 days and it had rained but was not raining. One loud knock from the rear. I was in reverse. My guess is the rotors got rust glazed from sitting in the rain.

 

It is common knowledge that our rotors rust like crazy. If you have the ebrake applied after washing the car and not drive it till the next day, the rust will lightly seize to the brake pads and when you first move the car, you break the rust. That's the loud noise you hear. If you leave the car parked for extended periods of time in humid conditions, it will do the same as when you washed it.