I use Lenovo Q190 2012 model. It was made actually as a streamer, because it's DEAD quiet and has SSD to make it even more quiet.
Recently installed W10, because W7 not in support anymore.
We've got some small booksized Lenovo PCs at work and they are also virtually silent.
My previous PC brought in 2004 was a Dell Dimension 8400 fitted with a Creative Audigy soundcard, and that was virtually silent too.
My current Asus PC from 2013 has an SSD, a quiet power supply and a low noise CPU fan, wireless keyboard and mouse, and so it's a shame that there doesn't seem to be any way of eliminating that headphone noise.
I intend to run W7 for as long as I can or until I can migrate to Linux. I use Windows 10 at work and can't say I'm a fan.
The noise is not terrible and many users wouldn't care but as we audiophiles understand, it's just not good. I suspect it is a motherboard interference issue as using the headphone jack on the back of the PC changes nothing.
As in the OPs case it is strange that the spurious sounds do change in volume and frequency without any obvious cause.
So perhaps someone with more knowledge of computer hardware could help identify the most likely possible causes here?