Found that the back tyre was flat when I was about to go out for a spin today. Got it pumped up so I could inspect the tyre but, eventually, found that the air was escaping (and quick too) at a spoke nipple. Anybody ever had this? Bike was only sitting a few days and I cleand the wheels, spokes etc after my last spin. Didn't notice any loose spokes...and the leaking one still seems tight. Question: would a loose spoke cause this? Or, is the wheel caput??