I did send it in. Nobody could hear a thing. As they told me, it was absolutely quiet.
It was the big toroid that hummed the most. The small one was barely noticeable. The 804C cd player also has a toroid. And the hum from that one is/was also neglectable.
I have had this trouble also and on the early Claymore's in certain location i.e. industrial or large town the hum was there and now and again I get same on some of the new kit. In some cases a DC blocker works, but in others the mains is to high and then you are screwed. For the Claymore we had Airlink (pt no 916) add 20% more winding on the primary and also the secondary, Problem in most case solved.
The new odd hum on again certain areas is due I found to mains borne coms from smart meters and my heart monitor communicating data over the mains, and this works often with either a mains filter or moving kit around on a separate ring main.
The other thing I notice with our new mains cable the interference from the digital garbage also disappeared but my cable is EMC loaded. (no I am not going tell how).
The cheap iron cores saturate you can often see this on scope and by looking at the voltage out and the load current it is easy to spot. In saturation you will see a current spike and often a flat top voltage, and the two can be seen at the start and the end of the distorted voltage wave form.
Have fun stay safe best Col ( put the earth back please)