My advice is to have it checked out by a certified mechanic.

Do you have a manual or automatic transmission? Your probably not doing anything wrong. Remember your boat and trailer are pulling away from your truck on a inclined ramp. Thats allot of weight pulling against the transmission.

JMHO

You need to have it checked.

Does it do it when on level ground?

If not, set your parking brake before putting the tranny in park. Your owner's manual will give instructions about putting the transmission into park on inclines.

From:

http://www.everything2.com/title/how+to+use+an+automatic+transmission

"In an effort to prolong the life our automatics and my arteries, let's look at the proper way to park an automatic transmission vehicle:

Step 1: Locate a parking space.

Step 2: Enter the parking space.

Step 3: With your foot on the brake, engage the parking brake, aka emergency brake or hand brake.

Step 4: If you are on an incline, slowly taking your foot off of the brake and make sure the car is not rolling. If it does roll, apply more force to the parking brake. When you know the car will no longer move, put the transmission in park. If you parked on flat ground, then you should need relatively less force on the parking brake. Just make sure you don't roll around.

Step 5: Shop 'til you drop, knowing that your car is parked correctly.

Step 6: Reenter your vehicle and turn the engine on while you keep your foot on the brake.

Step 7: Release the parking brake and choose an appropriate gear with which to exit your parking space. Notice the peaceful silence as you slide the gear selector out of park."

I suspect the "grinding" you are hearing is the parking pawl is jumping the teeth on the parking gear.

When you shift from park to forward or reverse, the gear may start to rotate before the pawl is fully disengaged.  I had a car that did this.  Sometimes on a steep incline, because the mechanism was worn, (and I did not set the parking brake) the pawl would jump out of the space between the teeth of the parking gear.  Then it would "grind" as the car continued to move.

"Park" should be used as a backup to the parking brake, not as the primary means of immobilizing the vehicle.

It is all too common that some drivers do not use the parking brake, depending exclusively on the tranny to prevent the car from moving.

He deleted his post , he must not have liked the answers.

I liked the answers.  I just felt stupid for asking.

I liked the answers. I just felt stupid for asking.

ON HERE THERE IS NO SUCH THING AS A STUPID QUESTION.