LT1Formula26

I was sitting in the driveway today trying to listen to this tapping noise I have coming from the engine that I have seriously had ever since I bought the car, so like 8 months. And as I was listening, I stepped on the gas to rev it and I heard a quick pop towards what seemed to be like where the header collector is...not sure though...but then I reved it again and didn't get it. I have had it happen only one time before when I was cruising at 20 and punched it. Also, when I rev it, there is a very small delay I hear (sounds like air) before the engine revs...what is this pop I heard?