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?
Had small GE one in our plant like that. We were paranoid, so we bought a new one and took the old one out of service - unplanned downtime on the machine it fed would have cost about $20K regen cost + $100K / day lost production, so it was worth it. Did a little "destructive analysis" and found that one of the core segments had worked itself loose. Upon de-energizing, the pieces would drop down about 100 mils. Re-energize, and the varnish was rough enough to keep it from popping right back into place. The vibration let the piece slowly work itself back into place. Glad we changed it out as the excessive movement of the parts had just about worn into the winding. Have also had similar sounds happen when the wedges that hold coils get loose. Good Luck!