I too have marks on my walls. We didn't know what was going on. We thought it was due to switching from semi gloss paints to satin or flat paints. Thinking that the walls surface had something to do with it. Ater looking at the heating units we wondered if the metal wire that runs behind to fins may be a factor. Perhaps this metal is deteriorating. Have you considered ceramic type units instead? I'm going to investigate these as an option. I was beginning to think we had a unique serious problem. We started to notice the dirty walls when our home was about 15 years old.