Sometimes when I'm backing up from my driveway, I push the brakes slowly and I hear a click noise coming from the rear, I don't think its the diff but maybe the rear brakes.

I ask my dealer and they mentioned something about its normal, the pads are adjusting or something.

Anyone lend some insight if you experienced this before? thanks.

 

http://smiley.smileycentral.com/download/t...75_ZNxdm799IGCA

The last time you stopped, you were likely going forward. Now you brake going backwards. The pads just shifted. There should be no surprises here.

 

This doesn't happen with other cars I've owned, why the S2000?

 

Rear caliper design. Each manufacturer uses a different kind of design. It just happens that the OEM Honda one has quite a bit of space fore and aft that the pads can slide around in. If you adapted S2000 calipers and pads onto another car, it would still click when you change directions.
Actually, cars with rear drum brakes will click also, but those are setting themselves when you brake going backwards. It's just a characteristic of brake construction and function. They're not all the same.
Another S2000 curiosity that few other 4 wheel disc cars have and that's the big crunch you get if you wash the car, set the park brake and park it overnight. No other car tends to rust the pads onto the rotors nearly as much as this car. It doesn't mean something is wrong. That's just the way it is.

 

It's pad shift, and normal.

 

My car does that as well.