Anyway in a moment of clarity I sent Dudley at Flexradio an email asking for his thoughts before I sent the rig into for repair.
Dudley got back to me within 15 minutes with the suggestion to try some other headphones. I went back to my Heil headset and no really noticeable noise.
Then after lunch got a second email from Dudley at Flex who, after talking to an engineer there, told me that Bose noise-cancelling headphones come with a sensitivity switch preset to high. The switch is on the plug that goes into the headphones and sure enough it was on high.
Turning it to low eliminated the noise.
This is fabulous customer service from a fabulous company that just happens to sell three of the best contest amateur radios on the market today.
One of the world-class contesters who lives near me in Ontario noted in his contest report on the Ontario Contest Club’s members-only reflector that he loved the way his filters in his K2 could work so close to adjacent big signals that often he didn’t even know they were there.
He’s right about the Elecraft radios (K2 and K3) as they are world-class but the Flexradios from the lowly 1500 at $600 and the 3000 at $1500 and the 5000 at $4,000 all come with a panoramic display built in.
Running a Flex at 25Hz filtering blocks out adjacent signals and the display allows you to see whether the big signal is up or down band from you.
After you’ve used a panoramic display, (which is a $600 option on the K3) you can’t go back to blindly tuning only with your ears!