A 1940s radio drama of "The Dunwich Horror," on the program Suspense. (I noticed they pronounce it "Dunnich," and not "Dunwhich," which is how I would have thought to pronounce it. What do you Northern's say?)
I was reading the Wikipedia article on this, and it mentions Greenwich, which is pronounced Gren-itch. I never really thought much about it, to be honest. But it seems like Dunwich would follow the same rules of pronunciation and be Dun-itch. Dunwich is a fictional place anyhow.