Maybe a larger explanation. Over time the bearings of any fan wear out. That is, the clearance between the fan shaft and the circular sleeve around it get wider. This clearance changes slightly as the bearing temperature changes, and is loosest when the system is cold.
On a new fan the clearance is so small you hear nothing unusual. When the bearings wear down enough, what happens first is that, on a cold start-up, the shaft rattles around in its bearing sleeve for a few seconds - may be a minute - until they warm up and the clearance decreases, and then the rattling action stops and it stays quiet. Even if you re-start with a warm fan, this noise does not happen. It only happens again when you start from cold. As time goes on and the...