ML 320 Catalitic converter noise?

While driving my 2001 MB ML320 from Portland OR to Bend, OR over Mt. Hood during a snow storm today I switched lanes and ran over snow that had collected between lanes during the storm, maybe 4 inches.

When the wet snow was thrown against the underside of the car from the tires suddenly there was a noise like a hole in the muffler or converter, close behind the engine, very loud in the passenger compartment. I looked under the hood and could hear it but couldn’t see anything.

Closed the hood and drove noisily away. after about 50 miles the noise stopped and by the time I got home it was sounding normal again.

Any ideas??

Answered my own question today. Went to the MB dealer and both catalitic converters have cracks in the welds, covered by recall in 2005. Must have been a very expensive fix for MB since 3 model years for the ML 320 and ML 430 were affected.
However, no one can account for the noise healing itself…