The expression is "bats in the belfry" – and there's a song for that. This is from Bang Bang, the second album by Dispatch, a group formed in 1996 by Middlebury College students Brad Corrigan, Chad Urmston and Pete Heimbold.
A belfry is "a bell tower, especially one surmounting or attached to another structure." The Merriam-Webster dictionary explains, "Surprisingly, belfry does not come from bell, and early belfries did not contain bells at all. Belfry comes from the Middle English berfrey, a term for a wooden tower used in medieval sieges. The structure could be rolled up to a fortification wall so that warriors hidden inside could storm the battlements. Over time, the term was applied to other types of shelters and towers, many of which had bells in them. This association of berfrey with bell towers seems to have influenced the dissimilation of the first r in berfrey to an l, and people began representing this pronunciation in writing with variants such as bellfray, belfrey and belfry (the last of which has become the standard spelling)."