Some were barely noticeable, while one in particular (a Droid Turbo) was so loud I could hear it getting ready to receive a call from another room. This was regardless of whether they were plugged in or not, although charger whine was its own separate issue.
Thankfully it does seem to be getting better over time- my current S8 is, as far as I can tell, genuinely silent.
There's a TDMA modulation frequency at 217Hz and this interferes with all sorts of nearby audio devices. CDMA and WCDMA phones have a much broader interference spectrum, which is why you don't hear it much anymore.