Why are CPU fans running on my MacBook Pro 15" full blast while my laptop has just booted up and waiting at the login screen? I know a lot of people have problems with the MacBook Pro's CPU fan running super loud and IMO the Activity Monitor is less than unreliable in telling what's really causing it… but let's check what happens when no applications are running.
What I find absolutely unbelievable that my Mac's CPU fans were running at maximum speed after just booting up, from a switched off state, with the charger cable disconnected. I can't see any reason for that, no applications should be running at this stage.
Is it possible to debug this somehow or is this simply an indication of a possible rootkit and a complete reinstall of my Mac?
Check your login items and launch agents.
Update to the latest OS.
Open Activity Monitor when you log in. See what is gobbling up your CPU.