is this normal?
 

I noticed when I'm either in 1st or 5th gear boosting @ about 7-10psi, a little after I let off the accelerator the car backfire's pretty loud. Does this sound normal? Below, I've listed information about the car and what I have. Any help would be great.

S5 vert w/ TII conversion
Knight Sports Turbocharger
Knight Sports ECU
Knight Sports Dual Pulley System
HKS 3" downpipe
Straight Pipe

Flamage is a trademark of the rotarys. You might check the decel valve on the tb. If your injectors have been upgraded to a larger size with nothing to control the extra fuel, you may be getting too much fuel, running rich, and dumping the raw fuel into the exhaust.

Timing?