I have been getting this knocking noise from my engine area from my pathfinder at exactly 2250RPM. It seems to only do it at this RPM and not above or below. It sort of is getting worse. Of course when on the highway at 75 mph that is where the engine sits. Since I have an extended warranty until 60K (I have almost 58K now) I took it to the dealer. I had a person ride with me and he heard it. He told me that it sounded like a heat shield. I have had heat shields on my other cars get loose and I know it didn't sound like a heat shield. Of course today they call and say its all done, come pick it up! It was a heat shield bent and they tried to charge me 135 bucks. I had a funny feeling they didn't have a clue what they are doing so as soon as I got it I drove it for a test and of course the knocking was still there. I love it when they lie to me when I ask did they take it for a ride and make sure it was gone? They reply it's gone! So we lift the hood and there is a piece on the top of the motor that opens up when the RPM is rev'd up and when it opens up it rattles. I really don't know what the piece does and none of the guys at the service desk know what the part is called either. Of course the service manager made me keep it there so his shop foreman can look at it and decide whether it is bad. I am assuming if it is driving me nuts, there is something wrong with it. My question is there anyone out there hear the same rattling. It seems that there is some intake door that opens up when the RPM's go over 2200 RPM. The problem is that these new vehicles require a PHD and the real problem is that the dealer themsleves don't have a clue how to fix them. Sort of ridiculous. So far, I am not really happy with my pathfinder. Nice truck, just has been in the shop entirely too much. I bought it new in March 2005 and has been there about 12 times in the timeframe for all kinds of crap from brakes (20 miles and a shim came loose and squealed like crazy) to tires (check those and you'll see that most pathfinders are wearing on the inside really bad with the BF goodrich TA Radials)and the rear hatch latch, tire pressure sensor, fuel vapor canister, recalls, idler pulley replaced twice, and the serpentine belt replaced twice.