Dont know if you've solved your problem but it almost sounds like an exhaust leak coming from where, for me atleast it was the smog tube the has a bolt that goes to the back of you engine block. I had to start my car and try to move this piece around and tighten at the same time till it didnt leak anymore. I wish I could describe it better but I still dont know what the piece is called. May help may not but hopefully it will.
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'86 GT cobra intake and plenum, E303 cam, 70mm throttle body, MSD 6al, gt40 heads, 373's and 2 chamber flowies.
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