Rene's question makes a great point toward my thoughts on this subject, generally-speaking. If the fan is featured onscreen and then shifts to being offscreen (and onscreen in the distant background), I personally tend to maintain the rhythm of the action without doing a hard-and-fast sync match if it's going to distract attention and we're doing a lot of visual A/B cuts - in the same spirit that if I cut a fridge run in the background of scene where the fridge itself is not important to the scene and we don't go near it, I'll just park it in one spot as one continuous region with no POV cuts for onscreen/offscreen; help out the mixer at the and of the day.
This rhythm-over-sync method takes a variety of conditional forms. A direct example is a computer screen with a blinking beep - and we cut back to it many times and the speed slightly changes (e.g. 8 frames apart instead of 5). In these types of situations, I'll take a least common denominator approach to rhythm-over-sync. Whichever onscreen shot has the longest and most steady duration of the beep, I'll cut to that first, and then mirror that speed offscreen. And if we come back to other shots and the timing isn't dead on as-is, I may do gradual, 1-frame incrementally exponential shift the offscreen beeps in between so that by the time they leave the end of the first onscreen shot, the sync up to the returning onscreen shot with no audible giveaway. When doing this though, one should take into account the rhythm of the sound so as not to "stretch" this sneaky effect too far to the point that the rhythm breaks.
Maybe an approach like this will work for your fan depending on how's been framed (or not framed) in the scene. Possibly roll up your sleeves and hand-build 2 full cycles of this speed change (designing fan blades can look similar in layout to doing heartbeats, if that helps visualize). Then, just re-use that as a continuous loop, making small timing cheats to the offstage moments so that the onscreen moments are dead sync, or at least a tight but slightly loose sync (because sometimes there's a time you have to trade off dead sync for perfect rhythm). Offstage can be your friend :)
Again, these are only my personal ideas and experiences to try.