I'm working in a 3 part ballroom and have been getting some feedback lately. The 3 parts can be combined i.e. 1+2, 2+3 and 1,2,3. The feedback comes even when I don't have anything plugged into the wall pockets in the ballroom. I have two questions. 1- What could be cussing the feedback 2- Where can I download different frequency sounds to help me id the feedback? i.e. 1k 1.6k 5k and so on so I can properlly EQ the feedback. Thanks!