You haven't provided a single clue about the noise so it's anyone's guess what the cause is. Is the fan motor rumbling? That's bad bearings and the sound will change when you change fan speeds. Do you hear a squeal under the hood only when you switch to defrost? That's a slipping belt that runs the AC compressor. Do you only hear the noise for a few seconds after switching modes? That's an actuator or linkage. Do you hear a hissing sound? That's a leaking vacuum hose or leaking vacuum actuator. The clue is the hissing will stop when you stop the engine.
The logical thing to do is have a mechanic listen to the noise. He will tell you if it's serious or minor, and he will suggest the best course of action.