I recently received the Lionel E9’s in Amtrak. A special run for Mr. Muffin. They run great and sound great. I have no issues. I am also 66 so maybe my hearing no longer hears those sounds.
I have the E9 UP set also from Mr Muffin. I have tested this set comprehensively and have detected no static or other background noise in the sound set. Of course, it's still brand new and might develop a fault, but it did not come to me with one.
That said, there was/is a distinct "buzz" in a couple of my early Legacy engines that was not present with TMCC - and these include two VL models, one being the Genset. I have raised this on the forum before and it's quite clear that the problem did not affect all engines in the production runs. I have wondered whether when the boards were encoded there was a mistake in some but not all, or whether the problem is really down to my transformer/control/home wiring setup.
Anyway, nothing of this kind is present in the E9s I have got and if it was I'd definitely hear it.