I agree that the problem really belongs to a competent sound engineer. The fact that you don't even know the frequencies of the noise is bad news from the start. The engineer should be able to prove where the noise is comming from before any repairs are attempted. Right now, it sounds like you are shooting in the dark. That can be an expensive and dangerous proposition. Locate a quality sound re-enforcement company and have them evaluate the system. You should end up with a report that says this is the problem and this is why it is a problem.
