Most furnaces make some noises when operating. A furnace, for example, makes a whooshing noise as the air travels through it and clicks when the thermostat reaches a certain temperature and sends a signal to the furnace to perform a particular activity.
These noises are normal. If you are hearing them, you do not have anything to worry about, however, weird noises may indicate a serious issue.
If you hear a knocking, whistling or screeching noise, there may be something amiss.
A knocking noise could mean that a part has become loose or air has entered a part.
A whistling noise generally occurs when the air inside the furnace tries to get around the filters, whereas a screeching noise usually indicates misaligned or damaged belts.