I had one on the bench where that was one of the complaints. It turned out to be a noisy op amp. Unfortunately the exact part was hard to find and I had to risk purchasing an overseas "equivalent" shipped direct from China on the slow boat. Fortunately it worked just fine. But that was something like five years ago. I can't say about whether the part can still be found.
I'm not a chip guy, but I've read that you can blast a noisy chip with freeze spray it'll quiet down momentarily. So that may be a good trouble shooting method.