Engine knocking in 2005 Off Road

HELP!
Brought my xterra in a number of times to dealership complaining of loud (windows rolled up) knocking that takes place up front of driver's side. The knocking takes place around 2500-3000rpm and at a km range of 55 - 100km/hr. It usually takes place under agressive accelation. The noise has been heard by both a service technician and the service manager. Now, I'm getting the runaround saying that warranty may have to clear the problem before repairing. I still don't know what the problem is ... they tell me it might be the heat shield. Don't know loads about vehicles, but a regular knocking is a heat shield???? I'm a tad skeptical. I'd appreciate any info./insight especially since I have to speak with the dealership in the next 2 days.

I also have a knocking/rattling sound outside the car in the situations you speak of. I don't normally hear it unless the windows are down. When I had the interior stripped down to the sheet metal (I was running wires and installing Dynamat), it was also very noticeable. I've always assumed it was a heat shield, and there is no knock sensor code stored in my computer.