Weird clicking sound

Iim the single owner of my 06 se 4x4 and it's got about 120k miles in it. For the last 50k miles I've heard a weird clicking coming from the engine compartmrnt. At first I thought it was engine noise but the tech at my dealer told me it wasn't, but also that he didn't know what it was. It comes from what sounds like some place directly in front of and below the heater controls. It happens totally unpredictably but only on acceleration. It seems to happen at higher elevation but I really have no idea... I live at 8000 feet in Colorado and drive all over the place at high elevation. Sometimes it will happen for half a tank of gas and other times I won't hear it for a month... But it's really loud. The tech at the dealership said he thought it was some sort of unbalanced thing behind the engine (I can't remember exactly) and that gets need to remove the engine to fix it... But he wasn't really sure. I have no idea what is going on... Sometimes it sounds like my car is about to explode! Here is a video: https://www.youtube.com/watch?v=5y4AcyzU-Y4

Anyone have any idea what this is?