Why is there a humming noise coming from the right, front bottom corner of my microwave when not in use?

That is because the power transforme is located in that area and being a transformer uses iron laminations to transform the energy from the primary winding to the secondary winding/s using magnetism caused by the alternating currents. This in turn produces eddy currents that excite the iron and cause vibration to be set up, (the principle is similar in a loudspeaker). That is where the noise is coming from.
Tightening up the laminations may help but it is normal for a small amount of humming/buzzing noise dependent on the manufacturing process of the transformer.
I hope that explains it for you.