I have a 08 F150 5.4 with 42,000 miles. I just installed a set of JBA shorty headers due to a broken exhaust stud on the passenger side. I was mainly hoping to eliminate the manifold issues in the future. Not a job I care to redo just for the record.
I now hear what sounds like an exhaust leak for the first 20 seconds after starting then it goes away. It doesn't make the noise any other time. Any ideas?

 

I have the same issue with my shorty edelbrock headers. Near as I can figure it's the headers not being completely flush with the block and when they warm up it closes the gap.