To cite Wikipedia on microwave radiation:
Microwaves contain insufficient energy to directly chemically change substances by ionization, and so are an example of nonionizing radiation. The word "radiation" refers to the fact that energy can radiate. The term in this context is not to be confused with radioactivity. It has not yet conclusively been shown that microwaves (or other nonionizing electromagnetic radiation) have significant adverse biological effects at low levels. This is separate from the risks associated with very high intensity exposure, which can cause heating and burns like any heat source, and not a unique property of microwaves specifically.
During World War II, it was observed that individuals in the radiation path of radar installations observed clicks and buzzing sounds in response to the microwaves radiation. Microwaves could cause the perception of sounds in the human brain by inducing an electric current in the hearing centers of the brain.