Update: So after a bunch of googling, fiddling, adding spot welds, the noise is still there. It appears the replacement crossover box is missing all of the internals that the MB pipe had. The outer appearance is almost identical. However the internals are missing. The pipes should go all the way through the "shell" and are connected with small venturi tubes. The outer shell is just that. A shield for support and a backup incase of an internal leak. The noise in the aftermarket crossover box is caused by the front pipes opening up into this box area. Because the internal volume is greater than the pipes, the exhaust is allowed to expand. the exhaust pulses are again squeezed in order to exit the box. This is creating an exhaust pulse collision/restriction and hence the hissing noise. These front pipes are the MB/Eberspächer style weld in replacements sold at SLS, Niemoller, etc. I do not know the brand name. I suspect the OP had success with adding spot welds because a small amount of volume was taken up with these welds. This was just enough to keep back pressure up and stent the noise. His engine is a 280. My engine is a 250 so there is less exhaust volume. Adding welds on 2 different sets of these pipes changed the pitch of the noise but did not fix the problem. Once again we have an aftermarket company attempting to reproduce a part and cutting corners witout testing. I would advise staying away from this part. Because MB no longer supplies this part, I will be making my own "H" or "X" pipe with the 42mm pipe size maintained throughout. I also have a set of pipes made by IMASAF that are a clamp on replacement style (at rear of crossover). The crossover portion is much smaller, however it too is lacking the exhaust pipe size, consistency, and internals. This set also has fitment issues at the manifolds. Will post details after a successful outcome of either.
