It's an 02 Bravada with a 4200 engine. Today I went on a 2 hour drive and when i returned and parked it in the driveway, my wife went out to go to the corner store and when she went to reverse out the yard i heard the strangest sound. It sounde like two pieces of plastic hitting together( like something in the fan ) so I open the hood and the sound was gone so I just waited for a minute or so and then this loud knocking sound started coming from the engine, or at least the front of the engine.. I was dumbfounded, the sound goes away as quick as it starts. It's not there all the time,it comes and goes. The oil pressure is fine on the gage, the oil level is fine, cooling is fine.. I hope it's not a main or rod bearing which is what it sounds like. If it was, would'nt it be there all the time? I cannot make the sound happen meaning , i've been trying different things with the throttle and so on but like i said,it comes and goes. By the way , I've driven it since and there is no missfire or loss of power or anything like that. Basically,it's just making this sound. Any ideas?