A loud scraping noise, a grating metal-on-metal sound, coming from inside your furnace could be a serious problem with the blower wheel. Maybe the wheel is coming apart from the shaft and scraping the casing or in some other way the motor is causing the wheel to scrape the casing. Turn off the air conditioning immediately and call an HVAC service company.
Fast Clicking/Flicking Sound
Remember that sound when you were a kid and attached a playing card to the spokes of your bicycle wheel? If you hear that kind of fast thwapping sound from your furnace, something might be stuck on one of the blower wheel blades or one the housing around the wheel. While your air conditioning may be working just fine, this stuck object is causing resistance and it will cause excessive wear on the motor. Get it cleaned up by an HVAC technician.
A squealing noise from the blower motor could be caused by a belt wearing out. That’s normally not an expensive repair and best taken care of before the belt stretches further or breaks.
Grinding Or Metal Groaning Sound
A bearing problem in the blower motor will make a dull repetitive groan. Some blower motors have ports for adding lubricant, but some are sealed. Adding lubricant will often eliminate the groaning sound, but you have to use the correct oil, not WD-40. An HVAC company doing an annual maintenance inspection on your system would do this kind of lubrication.
If you hear a rattling sound from your central air conditioning unit outside of the house when the air conditioning is coming on, the hardware around the compressor could be loose. An air conditioning service repair person should be contacted.
When operating properly, your HVAC system should be fairly quiet, but if you’re hearing one of these sounds you really should contact an HVAC professional to come in and repair the problem before it turns into an even bigger problem.