So, after my latest flight... One of the fans on my Nvidia 970M seemed to catch on an obstruction and stop spinning. I opened it up and cleaned it out, funnily enough the second fan that was still working was filled with debris, but the one that had stopped was almost spotless. So, running it with the bottom of the laptop off, the fan worked on standard and full rpm. It wasn't until I had put the bottom back on that I heard the fan ' catch' on something again and stop spinning, even when commanded to run at full speed. The case doesn't appear to be dented in any way, nor does the protective cover covering on the fans, so I"m not quite sure what the problem might be.
Looking forward to your insight.
If the fan itself is loose, due to damage, loose screws, bad thermal paste, etc., then this can happen. You would need to test to see if that happens again. If it works with it open, but not after putting the cover back on. If this is the case, then I would be looking at testing pressure on the fan when the laptop is open, to see if that makes it stop. If yes, then you need to determine which is the cause. Is the fan loose itself or damaged in its casing? Is it the thermal paste? Is it loose screws?
In the end it may well be something you can easily fix. If not, then you may need to replace the fan.