engine knock vs. trans knock vs...? help

[ericmuscle]

2010 camaro ss with 6-speed manual. i lowered the car and have flowmaster cat-back exhaust. my issue is when i accelerate through 1-4 gears i hear what sounds like a lifter knocking but i swear i hear it near the shifter area under the car. its a dull knockish type sound that increases with rpms(can hear it at 1,500-4,000rpm). i only hear it with the windows up, the sound is there whether engine is cold or hot. i can almost hear it at idle with the windows up, but after poping the hood and listening to the engine i dont hear it in that area. ive checked all around under the car for loose heat shields or something hitting the undercarriage, but i found nothing. any ideas??? anyone else got the same problem? im tempted to just take it to the dealer to get it looked at. not sure if there gonna give me a hard time about the cat-back and having the car lowering.....