When I had replaced my lower intake I had a similar noise, which turned out to be the egr tube. I just used a giant crescent wrench to tighten it more and the sound went away.
In your case though, you might have to take it back apart and put a new gasket in, if you get an intake leak that close to the heads your car is gonna run like crap.
2003 Mustang GT
K&N Cold Air/BBK 78mm Throttle intake/BBK long tubes/BBK cat less X-pipe/Flowmaster 40s