Could a be 1,000 problems. Bad ground, bad connections with the cover of the amp. bad coaxial cable and many other. You need to do a problem of elimination, then you,ll find the problem. I had a similar problem once where no one could find the problem, I pushed a 2 dB pad in mid stage and guest what, the suck out disappeared. This problem had been there for over a year. Does this make me a good tech..... no, I was just lucky to find it. You must do the same thing.
GOD LUCK.