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!
