street dreams

ok when my car is just idling a 800 rpms. and if u go to the back of thecar at the exhaust u hear a slight popping noise coming out of the muffler. now what is causing that. the car is the 1 in my sig. and also i started noticing at between 3000 and 4000 rpms when i am cruising on the road or on the freeway that in between that rpm the car seems to hesitate a little and my intake air seems to get louder like a all most whistle sound that i can hear on the freeway.
ok and i have no check engine light and i run rc 310 injectors and a msd digital 6 ignition w/ coil and my fuel pressure is set at 55 psi and i use a octane rating of 93

the only thing that i can think of is the dizzy might be going bad