I found that with my 3rd-party plastic phone case removed, holding the phone a certain way stopped the buzz. Realised I was bridging across the plastic that separates the top and bottom sections of the metal casing on the edges of the phone, about a centimeter or 2 up from the bottom.
This could relate to the grounding pins on the inside of the phone mentioned in other posts above, but at the moment I've got a bit of tin foil at that point to ground between the two sections of the casing and held in place by my phone case.
Testing sounds much better both ends, so will see how that goes. I'll open up the phone and do a proper job when I have time.