I have an intercom system that is running on 3 CT 475 amplifiers. I have recently notice a short blip/static sound coming through the speakers every five minutes. Not sure what is causing it. I hear it in all parts of the building and I have tried to narrow it down to a specific area by shutting down one amplifier at a time. No matter what the static blip is still there, unless they are all powered down of course. Not sure what else to look for? Any ideas?