Speaking of animals etc inside cases, I had one that happened to me personally, was living out in the country and we had a field mouse plague (those tiny little ones that can literally squeeze under a door)
Had a tower (one of the early 8 bay ones about a metre high) that had a server running on it, that after being running 24/7 for over two years, started making a loud squeaking noise- sounded exactly like a bearing in a fan was going (not surprising as I regularly would peel the labels off and innox the bearings from old fans and use them myself)
Undid the side cover (two pull latches) and open it up...
Yes it is a fan...
Or more specifically a field mouse whose tail had been dragged into the fan and had stalled the fan, trapping it with its tail jammed between a fan blade and one of the cross supports...
Still sounded exactly like a dry bearing, but it was the poor mouse complaining most bitterly..
Shame it wasn't a customers computer- would have loved to have put down the repairs done
"remove faulty mouse"