Are you engaging the parking brake when parking ? Sounds like it may be rolling up against a pawl in the transmission. I couldn't believe you have to do that even on flat ground but others here corrected me.

 

Nope I am just using the park in the transmission lever. I am not really sure about the times it is on level ground as its quite rare. the times its on slight or moderate slope and as far as i can remember always when the nose of the truck is down hill even then its a 1 in 100 type thing. the times my truck is parked on the top level spot of the driveway its often partially in ruts or sink spots from where the vehicle is normally parked and the ground has sunk a little. hence my suspicion its related to forwards pressure.

 

What @fdeno said is accurate - it is probably the pressure against the pawl.

When on an incline, it is best to stop, engage the parking brake, slowly let the truck roll to a stop, and then engage into park.
I have not had the loud thud, when putting the truck into drive when parked on an incline. I have done the similar method with my G5 Odyssey as well - never had that thunk.

 

I've heard sounds like that emanating from vehicle drive shafts when taken out of park when under tension. Kind of like when you tap a drive shaft tube with a wrench and it makes a very metallic reverberation similar to a bell.

Just throwing it out as a possibility.

 

It is my belief that this is an inadvertent effect of Honda's parking pawl design. There are other threads on the noise, and it is not considered an issue, rather just feature

 

I have the same sound coming from my truck and have been told that it is the drive shaft (Honda refers to this as a Propeller shaft). I hear it consistently when switching from drive to reverse quickly without pausing in neutral or when going from drive to park quickly without pausing in neutral. I also sometimes hear it when coming out of park into either reverse or drive. It happens consistently when parked on a flat surface although it is a quiet bell noise (tiny bell). It will happen every single time if I am parked at an incline without the parking brake on and shifting into reverse so that there is is stress on the parking pawl and the noise is loud enough to make others turn and look in a parking lot. I always use my parking brake now.

I have yet to remove my drive shaft and go through the same shifting procedures that would produce the noise to see if it goes away essentially proving if it is indeed the driveshaft making the noise but I plan to do so soon.