It's the transmission "park pin" disengaging. Use your parking brake to keep pressure off the Park mechanism. I use my p-brake every time I park. It also helps keep the rear brakes adjusted.

 

What I do when I park on a hill is...

1. park the car
2. keep foot on brake pedal
3. shift to park
4. apply e-brake
5. release brake pedal

When I leave

1. apply and keep foot on brake
2. shift to drive
3. release e-brake
4. GO!

they dont teach this in driver's ed?

 

(If your on an incline/decline)
1. park the car
2. keep foot on brake pedal
3. shift to neutral
4. apply e-brake
5. release brake pedal (Moves the weight load to the ebrake)
6. shift to park

When I leave
(same)

 

I was going to say ... People need to start using the E-brake but previous post did.
I don't get why people with AT never use the e-brake..

 

I agree. When I had my AT I always used the e-brake when parked on any hills. I admit that when I parked on a near flat grade I didn't use it but there is very little stress on the transmission. However, if someone were to hit the front or rear of the car, without the e-brake on, it could also cause a lot of damage to the transmission.
When I took driver training (1981) they taught this and I know for a fact that they still do today.