We definately need to know if it is on all legs or not. I know you don't want to take the plant down. But I would maybe schedule a planned outage at your worst offending amp so the call center does not take calls and verify what leg it is on first. If it is on multiple legs, you can bet it is AC on your sheath. Then you will have to break each leg off the housing to find out which way it is coming from and pinpoint it that way. Time consuming I know, but it is a very thorough and you know for sure where it is coming from.
This is CABL.com posting #169005. Tiny Link: cabl.co/mR73