Having some problems with my engine when I accelerate.. When I step on the gas to accelerate it sounds like its bogging?? Like a thumping sound when I try n speed up.. If I just drive normally there is no noise.. I had problems before with the air fuel ratio cuz I need a tune, the gauge would read lean while it would make that sound but not when I would accelerate and it would just last a few seconds.. Any ideas dudes? Oh ya and its a 1996 GT with a swapped 03 GT engine..