Common signs of HVAC system problems include strange noises coming from the system, poor air flow, uneven temperatures throughout the home, and higher than normal energy bills. If you notice any of these signs, it is important to contact a professional HVAC technician to diagnose and repair the issue. Other signs of HVAC system problems include a musty smell coming from the vents, visible condensation on the walls or windows, and a lack of hot or cold air coming from the vents. If any of these signs are present, it is important to contact a professional HVAC technician as soon as possible to prevent further damage to the system.