It has to be a leak of some sort. When you bolted the catback back up did you evenly tighten the bolts from the catback to the H-pipe? And same with the bolts from the headers to the H-pipe? Do you have a gasket in-between the headers and the H-pipe? check all of those and see if it still does it.
I have BBK full length headers and an o/r H-pipe with mac catback and I have no hissing noise like you described.
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88 Stang
306 ci.
GT Mustang heads- ported and polished/milled---Soon to be AFR 185cc
1.7 crane roller rockers/extra hardened pushrods
FRPP X-303 Cam
Stock Intake-1 inch intake spacer
BBK Cold Air Induction
Stock Injectors--going to 24 pounders
BBK Full Length Headers
Mac 2.5 inch Cat-Back
8 Point Cage
Sub Frame Connectors
World Class T-5
Steeda Shift Cable,Quadrant,Firewall adjuster
King Cobra Clutch
Pro 5.0 Shifter
Alunimum Driveshaft/ Drive shaft loop
Kenny Brown suspension
3.55 gears
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