My '79 450SL does the same thing. Sometimes it's more noticeable than others. Like with the top down. I've been told it's the fan. Mine has a clutch fan. Don't know if yours is the same, but I would think so. It does get irratating at times, but I don't think there's anything that can be done.

sbr
'79 450SL 75,000 mi
milanbraun/palomino
lake st.croix beach, mn.