I've had similar issues more than once - the first time - I narrowed it down to the cable tv box - so I just don't have it hooked up anymore - but to nail it down - I removed every component from the system one at a time - power as well as any ground or interconnect that connects any single component to the system - this at least identified the culprit component.
The second time was a lot more complicated - i had an annoying hum in one speaker ( ml Clx ). I switched my mono blocks from left to right - hum stayed with the speaker - i switched boards from one speaker to the other - stayed with the speaker - removed all Inputs from the amps - still had the hum - now I was convinced that I needed to buy a new panel - and was going to swap the hf panel from left to right to see if it moved with the panel - or stayed with the speaker - I had just bought a new rack - so was plannning on tearing it all down and rebuilding the layout - getting rid of any redundant wiring connections - example - having one source hooked up to two different dacs - or to the same Dac in different ways - USB as well as spdif etc. so I go rid of a lot of redundant cabling - I took each component into my workshop - took the covers off - blew everything off with an air compressor - then spent time cleaning every connection on the outside of the chassis with deoxit and pipe cleaners - then again using the gold version of deoxit - do this on all your IC's and power cords as well
Put it all back together - and the hum was completely gone -
im just suggesting that although it's a lot of work - cleaning the connections is necessary maintenance at some point anyways - and by doing this - you can pull out - and put back in - each component one at a time to nail down the culprit ( if it happens to be a single component - and benefit from knowing every component now has no oxidation issue that might be contributing - and when putting it back together
The second time was a lot more complicated - i had an annoying hum in one speaker ( ml Clx ). I switched my mono blocks from left to right - hum stayed with the speaker - i switched boards from one speaker to the other - stayed with the speaker - removed all Inputs from the amps - still had the hum - now I was convinced that I needed to buy a new panel - and was going to swap the hf panel from left to right to see if it moved with the panel - or stayed with the speaker - I had just bought a new rack - so was plannning on tearing it all down and rebuilding the layout - getting rid of any redundant wiring connections - example - having one source hooked up to two different dacs - or to the same Dac in different ways - USB as well as spdif etc. so I go rid of a lot of redundant cabling - I took each component into my workshop - took the covers off - blew everything off with an air compressor - then spent time cleaning every connection on the outside of the chassis with deoxit and pipe cleaners - then again using the gold version of deoxit - do this on all your IC's and power cords as well
Put it all back together - and the hum was completely gone -
im just suggesting that although it's a lot of work - cleaning the connections is necessary maintenance at some point anyways - and by doing this - you can pull out - and put back in - each component one at a time to nail down the culprit ( if it happens to be a single component - and benefit from knowing every component now has no oxidation issue that might be contributing - and when putting it back together