I have searched here for a similar issue and did not find what I thought was the say issue. I have a 2012 F-150 Super Crew 4X4. I am just shy of 90k and this noise is only for about 15-20 seconds on a cold start. It is a hissing noise that reminds me of an exhaust leak (cracked manifold) and after about 15-20 seconds it is quiet. Like maybe the manifold can swell up and seal off?????

Any and all suggestions will be appreciated....