99 cobra, 4.6L 4 valve motor with 93000 miles. Shows rock steady idle vacuum (21 in hg), compression is good on all cylinders (180 psi). The car has a mac cai, steeda oil separator, bassani x with cats, and magnaflow catback when i bought it. When I get on it in any gear, from about 3000 to 6500 it has a hiss like it has a turbo making boost. It doesn't tick at idle so I don't think it is an exhaust leak. all bolts are tight. It sounds like it is coming from the driver side front. I originally thought the cats were starting to plug but the idle vacuum remains steady and doesn't trail off. Has anyone encountered this before?