Maybe. I was also wondering if it was caused by two different metals (aluminum head and cast iron manifold) and their different expansion rates.I was thinking, could this warping be due to the hot side of the turbo being too close to the back end of the manifold?
I don't hear it when my engine is stone cold or at proper operating temp for at least 15 minutes. It's most noticeable after the truck has been sitting for a few hours, I start it and I give it more than 1/3 throttle at a start (or low speed). It probably is initially the gasket.
Unfortunately, from various searches on the Internet, it seems this is fairly common on other later Ford engines - 5.4 most notable. Maybe it has something to do with Ford's quality control on cast iron exhaust manifolds.
I checked around for aftermarket tubular exhaust manifolds (Full Race) but their manifolds are designed to relocate bigger turbos, require their special Full-Race DP (not their stock replacement), and need their CAC. Full Race does not currently have a stock replacement tubular exhaust manifold.
I am giving this serious thought as I hate to go through the "time & expense" to replace the stock cast iron exhaust manifolds only to have them "fail again" in the future.