I had this on my 1997 155. There is a little heat sheild spot welded in 4 places to the downpipe just before the cat. Two of these welds broke and I tied the rest on with stainless steel wire. Then the other two welds broke and I took the whole lot off and have had no side effects.
The other thing it could be is the plastic under tray for the engine if your car has one. This often drops down and catches on things.
I also have that noise. My exhaust is brand new, removed cat, don't have that thing on the down pipes, and also not plastic undertray.
What about that small box just after the lambda sensor? Couldn't that be it. Although tapping mine with the hand doesn't make any noises. Maybe I am not hitting hard enough....Anybody know what that is by the way? If it is a pre-cat, is it also restrictive, or not?