Can't seam to kill this damn leak...

So I just got my car back from getting a new motor put in it at Ford (excellent job, BTW), and I still have a slight exhaust leak on the passenger side where the x-pipe meets the header. It has a new gasket in it and everything, but I still hear it. You can't hear it at idle, but in the low RPMs when accelerating, you can hear the sound of air (pretty quiet though). It's not a loud leak by any means, and it's not making that annoying rattling noise that most leaks produce. Any ideas as to what I can to? I'm assuming that the bolts are as tight as they can be since Ford just reconnected them to the headers, but I dunno. I'm starting to think maybe the upstream O2 sensor is a bit loose. I'll have to check it out this weekend.

Any input is greatly appreciated, thanks.

Yeah, aside from this minor sound, its running amazingly. The motor has about 350 miles on it, break-in is until 1,000 miles. It's so painful. I can't wait to let her loose!

BTW - I had this slight leak before the new motor, just to clarify. I'm thinking maybe the Pypes O/R X doesn't fit that well.