Hi ThermalMike,
Excellent call Before I read your reply I was suspicious of the same idea; took a pair of needlenose pliers, and gently pulled down on the grill just enough to "bend" it away from the fan and the noise stopped. It came back later when I had GTA V going on full blast, so I did it again but a little bit harder, and it hasn't come back since, and it's been about a week now.
My concern now is, it was tucked away, completely enclosed before and when it happened, and nothing is/was loose inside the case that would've pushed the grill up into the fan. What do you think might've been the cause of the fan moving toward the grill enough (or vice-versa) to cause the rattle, without any external cause?