I have the ET3 and a very similar situation. It drove me crazy but found the source -- dimmers in other rooms. I thought it was an issue with pass-through but found that if I turned the volume up while on any source with pass-though disabled (not playing music or anything of course) the buzz is the same at volume level 66 which as I understand is the same as pass-though (unity gain).
Even if you don't have dimmers checking the above could tell you it's not a problem with the ET3/pass-through itself but probably an outside source only making itself known at high volume levels.
Even if you don't have dimmers checking the above could tell you it's not a problem with the ET3/pass-through itself but probably an outside source only making itself known at high volume levels.