Yep...My 92 400E did the exact same thing. I plucked as much as I could from the center vents then went on a mission to find the source. I pulled all the plastic molding and rubber from the bottom of the windshield, removed the wiper assembly, removed the top to the fan/blower and exposed the blower motor and fans. On the metal baffle that seems to control fresh and recirculating air for the cabin was a sticky combination of glue and decomposing foam. I took it off and cleaned the baffle.

Once I reinstalled all the parts I operated the fan on "High" for a moment which seemed to "process" the final pieces of foam out of the center vents. Since that surgical procedure...no more blowing foam.

I am not sure of that's the only source of decomposing foam in the system but I can tell you my problem went away.

Good luck.

JC

__________________
Jeff
'87 300E
'92 400E
'06 ML350