The fan is now humming, what to do?

My Mac Mini (Mid 2010) has suddenly developed a hum it has never had before. I did a complete teardown to determine the cause. Its not the HDD, nor the optical drive (which I replaced with an SSD)... It is the fan! But why now?

In the process of reassembling I found a torn rubber bolt gasket, like the kind you see in the three bolt holes of the fan unit. Since all those were intact, I don't know what piece it came off of. I didn't see anything else in the teardown that had one of those... and yet here is a torn one, and I have a rattle. Anyone no where it might have come from?