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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?

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    Boot in safe mode (hold SHIFT until you see the Apple logo) if the problem goes away something that was installed is doing this. If the problem doesn't go away I would take it in for service.
    – Steve Chambers
    Commented Aug 27, 2020 at 15:03
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    How long after logging in did this occur? Your machine could be running scripts.
    – Natsfan
    Commented Aug 27, 2020 at 15:39
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    Do you have access to a second Mac? If I use Terminal's ⇧⌘K (Shell > New Remote Connection...) and connect via ssh, followed by running the top command (top -o cpu), I see mdsync using quite a bit of CPU percentage (not enough though to be problematic, some 20%). "Remote Login" in the "Sharing" System Preferences has to be enabled on the Mac with the issue.
    – Redarm
    Commented Aug 27, 2020 at 17:05
  • @Redarm It's a very good idea to connect via SSH, I will try it, thanks! jmh: The fan speed went away I would say in a few minutes after logging in. Also this does not happen each time I start my mac, but occasionally it does.
    – adamsfamily
    Commented Aug 27, 2020 at 19:20
  • @SteveChambers Unfortunately it behaves the same in safe mode, too. Redarm: I am not able to ssh nor ping the machine during login screen (as on my screenshot). Maybe it's due to FileVault but definitely no network until login.
    – adamsfamily
    Commented Aug 27, 2020 at 19:39

2 Answers 2

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  • Switch OFF FileVault drive encryption; or login after restarting/shutting down. (Tested)
  • A clean install MAY help. (Haven't tested that)

Same problem with my '15 Macbook Pro with Monterey. This has started happening after a few recent macOS updates. I've read elsewhere it's something to do with a runaway FileVault system process.

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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.

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    (I have the same comment as below) Are you sure that Login items can do anything before I first enter my password? Since I'm using FileVault, macOS should not be able to decrypt any of my files, it shouldn't know anything about Login items IMO.
    – adamsfamily
    Commented Aug 29, 2020 at 16:04
  • I’m not a coder to give a definite answer, but some processes start up already when you press the ON button, so there is a chance some of them are preparing login items, etc. Login screen is just a visual interface, but Mac scripts run in the background. Not all processes have to have access to your data to run, imagine them as empty containers that are getting ready to be filled with your data when you login.
    – Pavel
    Commented Aug 29, 2020 at 16:44
  • Besides, encryption prevents unauthorized users from accessing your data. I don’t think Mac prevents itself from running scripts on your data, at least not in all cases. There’s distinction between ‘accessing data’ (reading it) and ‘running scripts on data’ (moving it around without looking what’s inside).
    – Pavel
    Commented Aug 29, 2020 at 16:48
  • This answer cannot be true, the FileVault login screen appears before macOS has booted since the disk is encrypted. This preboot is a separate read-only volume.
    – grg ♦
    Commented Sep 28, 2020 at 15:33

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