Honda BF135A Question

I am running a 2006 Honda BF135A with around 2000 hours on it. Lately while going into the forward gear it goes in then makes another loud clunk like it is coming in and out of gear. It also makes this sound when coming down off plane, sometimes more severe than the first example. Anyone here ever experience this or have any idea what it might be. I was thinking maybe the shift cable is going out. It run's fine on plane. No noises coming from the lower end. I have had this motor since it was new and it is mounted on an 18ft Design Concept boat.

My first instint would be to thoroughly check the mounting bracket on the transom and your tilt/trim hydraulics. From what you described it sounds like your motor may be riding up on the pivot bar, then reseating itself when you put it in forward gear, or are coming off plane. Easy to check. When you first put it in gear, observe the top of the engine and see if it moves into its seated position.