Had that on the 190E we aquired, and found a loose heat shield intended to be tack-welded over the exhaust pipe, just south of the manifold downtube. I think the pipe may be a 'pre-cat'.
On other cars, have found similar things, but sometimes it is from an internal exhaust baffle. The only solution there is to replace the offending component.
As far as diagnosis, if internal, it may be very difficult to find if the car needs to be under load, because someone needs to check underneath for it. Can you get it to happen in 'park' with lights, a/c on, foot on the brake, etc?