Undesired friction can occur if a component within your system is soft, broken, or out of place. It will produce noises such as pounding, clanking, screaming, humming, buzzing, etc. It may also cause additional component damage.
Unusual sounds can also indicate an electrical fault. Pay careful attention if your air conditioner or heating system creates unusual noises. In addition, before the problem worsens, you should shut down your system and consult a technician.