It's "ano" , not "aano".
"Ano niwa ni nani ga arimasu ka" means "What is in the garden?", as Morphling-san translated, whereas "Ano niwa ni nani wa arimasu ka" is asking if there is "that thing" in the garden. "Nani" is a pronoun for something inappropriate to say or the speaker don't remember the name here. The intonation is different from the interrogative "nani", though.