You mad fish. Ours used to do this. It goes "ding dong" (classic, understated, elegant) and then odd times it would start doing the bl**dy Big Ben chimes (tasteless, Hyacinth Bouquet, embarrassing). Dh did that annoying man thing of - not having heard it himself - telling me I couldn't have heard it (aural hallucination, spending too much time with small children, obviously unhinged). Then it did it when he was in and he started spouting on about wireless radio waves or somesuch.
Never occured to me I might have a haunted doorbell!