Chasing the gremlins can be a real pain. I once had bad channel (out of 576) in the SONAR aboard the ship. I finally found a tube that had cracked and let air inside the envelope, but it still glowed! (We used to look for blown tubes by turning the lights out) I found that little sucker, I think it was a JAN Phillips 12AX7WGT, and strung it up on the bulletin board in the Control Room. It swung there for the entire cruise and everyday I laughed at that sucker.
You will be overjoyed when you find the issue. May I suggest the halfway approach, where you look for the noise in the middle of the circuit (disconnecting parts to isolate the "middle"), if you find noise, go back towards the source. If you don't find noise, you go forward halfway again and keep doing that until you can find a small section where there is noise on the output but no noise on the input (with isolated components, as required). It usually works for me.