Possibly the antenna is now close enough to the rack equipment that low level signals/noise are mixing in with the wireless signal to the antenna. For this to happen usually there is either a gap in the shielding or a grounding problem. Your options:
1) Pull the transmitter from the rack as planned.
2) Try a different cable between the transmitter and the antenna. If the cable and antenna are one piece, then try a different antenna.
3) Try turning off single pieces of equipment in the rack to see if the interference stops. If one or more of the rack mounted pieces of equipment can be identified as the source(s) of the interference, then check the cable(s) coming out of that equipment. The best way to be sure about the cables is to replace with a known good or new cable.
4) Sometimes an interference signal/noise travels along an electrical ground. A power conditioning unit that can handle all of the equipment in the rack may eliminate this. Before buying such a unit, try using an extension cord to supply power from an outlet on a different circuit breaker to the transmitter.
5) Just like shielding the cavities of a bass guitar, sometimes external shielding for the equipment can help, though this will be a trial and error process.
6) A more sophisticated approach uses a spectrum analyzer to determine the frequencies of the interference and then check to determine the source of these frequencies. Someone good with an oscilloscope may be able to accomplish this.
7) If a single piece of equipment in the rack is the source of the interference, then consider replacing this equipment.
I hope one of these options does resolve your situation.
The church where I play bass picks up interference from my bass whenever I use a Tech 21 Bass Fly Rig. I never have this problem anywhere else. I suspect the overhead lighting to be the source of this interference but have not been able to verify this as it requires someone to turn on the sound and lighting equipment, then to turn on the lights in various combinations. Shielding the cavity of my bass guitar did reduce overall noise but not eliminate it. Using different 1/4" instrument cables made no difference. Likewise, trying different XLR inputs to the sound system made no difference. Using the latest version of the Tech 21 Fly Rig made no difference. At this time, I simply go straight from the bass into a passive DI box.
