Dear car owners, yesterday found that turning on the air conditioner ac can listen to the engine compartment Oh Oh, buzzing this sound, like what is the mechanical rotation, sent out, turn off the ac immediately gone,

Dear owners, yesterday found that turning on the air conditioner ac can listen to the engine compartment Oh Oh, buzzing this sound, like what is the mechanical rotation, sent out, off the ac immediately gone, did not pay attention to before, this is normal? 4s said that the mechanical operation is going to make a sound, I don't know if this sound is normal, can you hear it in the car under special quiet, feel the sound of the compressor or the fan?