From what you describe the rumble is you lugging the engine and when you floor it it'll kick down and get into the power band. Remember, a 96 Automatic GT is not really a fast car at all... in fact most stuff that comes out nowadays will be faster than you are. You're only at 215fwhp and probably more like 160 to the wheels if its stock...
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.