The head fan is the approximately 1" diameter fan that you can see as part of the head once it is removed. Didn’t know that they enabled tachometer readings on that one for monitoring. I found this post from Support about cleaning that fan. Personally I would pull the mirror and the head lens first so as not to deposit gunk on those with the compressed air. Though I guess if the lens is pushed all the way up it will block gunk from getting on the upper mirror and the instructions say to clean the lens as a last step.
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