I worked on my sister 2000 V6 AT accord today. I replaced the rear pads and rotors, I don't think his is related to it at all. When I was done I needed to move the car out my garage. When I shifted from Park to Reverse, you can hear a hissing sound. It is an old car with over 220k on it. The transmission does make the hard clunch sound when you put it into Reverse but this is the first time that I've noticed the hissing sound.

The sound is gone when I put it in any other gear. The car is NOT moving or anything, I still have the foot in the brakes.

 

I don't know for certain, but make sure that what you are hearing is not just the vacuum power assist for the brakes.

 

I've never heard this sound before. If it is, is there a way for me to check? I never hear of this before.

 

I would set E brake, have someone inside the car while you are listening under the hood (near the power assist) and first have them try the brakes, then try the brakes/reverse. I am not sure of how you would test it mechanically besides if it looses vacuum assist within about 10 seconds of the car being turned off (no foot on the brake pedal when the car is turned off to see if it holds vacuum).