Popping noise under hood
 

Ever since I installed my injen intake a couple of months ago I have heard a popping noise from under my hood a few times, almost like the car is backfiring. It seems to be happening when I drive a steady mid range rpm. Places like parking lots where I'm on and off the gas and my rpms stay steady between 2 and 3 grand. What the hell is causing this? Help please.......