I was called to a store at which a large number, (maybe 25%), of the fluorescent ballasts began buzzing all on the same morning. The problem is both on the sales floor, where the fixtures are triple ft40 troffers and also in the receiving area where the fixtures are f32 t8s. All the affected fixtures appear to work like they normally would, with the usual brightness and no flicker. Can anybody suggest what might cause so many of the ballasts to suddenly begin buzzing all at the same time?