Panasonic Microwave buzzing when door is shut and not in use!

espresso

Avoriaz wrote:

Again I suspect transformer buzz.

With the door shut the load on the transformer is at a low level that causes it to buzz. When you open the door, the light comes on, the load on the transformer increases and the volume of the buzz changes, reduces or disappears below audible level.

Very unlikely!

Transformer buzz is not a "high pitched buzzing noise" being 50Hz and the interior lamp is operated from the mains voltage.

Stompa

If you find it irritating then I'd suggest taking it back.

Alternatively, you could do what I do and only turn it on at the mains when you're actually using it.