At one of our schools we did a complete shutdown to replace a burned set of fingers in the main gear. No prob there. The prob/"unusual event" came when we energized. One of the transformers, a 75 kw 480 - 208, 30 year old Zinnsco began to buzz like the devil. It was so loud it sounded like one of the old door bells (when they used to be buzzers not bells). After I finished panicking I put an amprobe on all the phases on the hi side. It was only drawing about 17 amps (minimal load due to the school being closed for the holiday break). The voltages on the primary phases varied but were within ~6% of each other. Within 20 min it suddenly quit all the noise, went into a "normal" state and was fine. We then shut it down again, opened it up and checked everything we could get our hands on for tightness and wear; found nothing unusual. Even verified the grounds, no prob. When we fired it up again ... same thing. But 20 min later it was like somebody threw a switch and it instantly went into run mode. Anybody every had this prob? Any suggestions?