shudder/flutter in dash area?

My 1983 380sl has a very noticeable "shudder" or flutter in what seems to be coming from the dash area. It is more than a leaf, etc. in the ducts. It doesn't happen continuously, but seems to occur after the car has been run for a time. Any ideas?

Hi Paul. I had the exact same problem. I brought it to the dealer and they said I had an aftermarket fan shroud that was causing the problem and they replaced it with OEM. It was fine for about 2 months, but now its happening again, louder and more frequent than ever. Its back at the dealer today. I will let you know what they say this time

Paul, as it turns out, this sound is coming from our transmissions. My transmission runs great except for that occasional flutter. They tell me the transmission needs to be rebuilt or replaced. That fluttering is a precurser to a transmission problem