Why does one hear a hum when standing near a power tranformer?

Why do transformers hum?

We could use the tired saying 'because they don't know the Words,' but that might get us sidetracked.

The short and simple answer is that Transformers hum because of an effect known as 'lamination rattle' caused by DC voltage on the line or poor construction or both. 'Lam' rattle occurs in all transformers to some degree, that degree being related to the quality of the transformer and the quality of the line voltage.