Phil: thanks for your comments. Yes, I am grasping at straws - I fully realize that, and as I somewhat expected, replacing the input caps did nothing to alleviate the noise.

I have checked the AC connection to ground and that is all OK.

And I have also attempted some time ago to use your trick of a small SW receiver to attempt to find the noise but cannot pin down any one component as the culprit as the noise seems to emanate from all over the output section.

I am loath to start replacing any more caps until I really determine where it is coming from...