My furnace is making a very loud noise... what should I do?

Operation of unsafe unit is highly prohibited. Even though this could be something simple it can lead to something more serious. You should hire a professional to diagnose the issue correctly and provide you with repair suggestions. If your equipment is less than 10 yrs old AND if its registered most likely you can claim the failed part under warranty.

Yes, I've seen many furnaces make loud noises. It could be a number of things. It could be a bad fan, a bad fan belt, bad bearings, or else it could be a bad Ventura. Motor usually when a bad ventric motor is going, it'll start making loud noises. Also a good idea to have a clean filter as that will also cause the problem. In this case, there is more than one issue that it could be most likely it'll be one issue. But there is three or four causes of it

Start by changing your filter monthly and never use high merv rated filters 8 -- 13 Merv. These filters are restricting airflow and the fan is likely pulling the return air duct inward when it starts decompressing the duct and then expanding the duct when the fan goes off causing that noise. If that doesn't resolve the issue then you likely have a motor issue on a newer furnace or a Combustion issue on an older furnace. Regardless, contact a licensed , insured HVAC contractor in your area