Bob, it's not a "toughie"
We have dealt with this problem for years and I can assure you that the 624 is merely amplifying
the problem.
It's being induced either from the field wiring, the AC or the ground or a combination of any of these.

OhioTelecom and myself are simply trying to help you out.
I would suggest at this point you call a technician who deals with these problems on a daily basis
So i'm going to leave it at that and close this out.
Good luck!
Gem