It just sounds like it's coming from the exhaust, really it's a backfire in the engine. The H pipe with cats/precats echo the sound. It's not a good thing to have them doing that btw, you can blow your converter honeycomb to bits.
Well, the fuel pump relay is under your seat. There is more the the fuel pump circuit than just that though. The fuel pump itself could be weak. I'd give it a fuel pressure test and see what that looks like. The fuel pressure may be low, which would cause your car to run lean and "pop".
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