Is it a desktop or laptop ?

If desktop-
the power supply is in the back of the case usually the upper area where the power cord plugs into.
You can listen in that area to see if the sound is worse from there.

Hard drive is usually in the lower front area.

Possibly could be the fan area and air holes need cleaned out?
Esp if pets in the area shedding and such.

I don't know if you have ever opened the case to clean inside- mainly just careful dusting/blowing out - If you decide to do that shutdown & unplug power cord.

If you decide to open it -
Might be good to google some videos of how to do it, easier that me trying to type it all out. mainly avoid static, and don't bump, jar or overly disturb any inner parts.

I have at times plugged the power cord back in with the case open -to start the computer and listen to locate the odd sounding part.
But I learned on old garage sale puters, so didn't matter if I messed up...