That's attenuation noise ... if the radar isn't picking up any rain echoes, the operators increase the sensitivity ... literally dust and pollen ... (but not grass) ...
The radiowave and microwave portions of the EM spectrum are regulated by the FCC ... and the 10 cm band is assigned to the NWS for weather radar ... just happens to be microwaves is all, nothing special ... as harmless as radio ...
The analogy to a microwave oven isn't quite correct ... in the oven, we're concentrating energy ... radar disperses the energy, half is in space within a single thousandths of a second ...