Could be a number of things.
You could have a fan dying, you could have something that has dropped into a fan, could even be the fan on your graphics card.
I would open it up and see if you can’t locate the source.
This will require having the panel open while it’s running so you can troubleshoot. Don’t worry, computer techs do it all the time, it won’t hurt it to have the side open for a bit while you look around.
Just be careful which things you poke so you don’t break something or short something out.