Exhaust Leak!!!

I just swaped out the heads, intake, etc on my 95 GT, I used ffrp ceramic coated shorty unequal length headers... Everything went together fine, but appearently I have an exhaust leak , it makes this funny sound, I don't know how to describe it and the mechanic said it was a leak where the headers meet the heads... I have retightened them twice and still makes that sound, what should I do???