Ah, well there you go - an AC transformer. As I said, AC devices can vibrate. From what you say, 115VAC is fed to a small transformer next to the light which steps it down to 28VAC to feed the light. I am guessing now, but perhaps the light, (which is external and subject to the weather), is fed with AC rather than DC to avoid electrolytic corrosion problems?
A transformer works by magnetism - the magnetism generated by one coil induces a current flow in the other coil. If the windings in the transformer are loose, the AC magnetic field will make the windings vibrate more than normal, giving rise to an audible sound, (same mechanism as a loud speaker, but obviously unintentional and undesirable in a transformer).
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Last edited by Uplinker; 15th May 2013 at 06:06.