A transformer buzz is generally to be considered acceptable. Simply means the lacquer between the laminates is getting brittle. At some point beyond most of out lifetimes, the core laminates will all go short, and the transformer will overheat . But it's on for such a short time anyway, I'd give it no further thought. Wait for E von Hoegh to chime in on the thread. If he refutes me, I stand corrected.