So I picked up (CL curb-alert!) this hybrid/comfort
Giant Farrago, and I got it running OK for my wife, but there's this sound coming from the rear hub that's like a teakettle. Once in a while it will stop, but mostly its just droning on with this high-pitched whine, sometimes it changes pitch cyclically with the wheel rotation (like a little fire engine siren: wee-ooo wee-ooo wee-ooo...)
What could possibly make a wheel sound like that, and what can I do about it?