We have a carrier gas furnace which is only about 5yrs old. The fan is not turning on the unit outside and it has begun making a loud noise but its working fine as far as heating the house. What could be the problem and would it cost a lot to fix. Thanks for any help.

It is possible you have a heat pump, which may have an outdoor unit with a fan , and a compressor that can make a loud rapping sound. The heat you have may be coming from auxiliary heating units, usually electric.

Any loud noises usually means something mechanical is broken or worn, and unless you can work with tools an understand what you are working with , may need a tech to look at.