My anti cloud was making noise

I was making myself something to eat and I heard a loud humming sound from my anti cloud. What is loud to me is dismissed from my wife and my cat. They pay no attention to the sound and go about their day.

This enclosure has 5 disks and 3 out of the five spin at 5400 rpms and two spin at 7200 rpms. The two on the right and the two on the left are pretty close in size and the one center mass is smaller in size. Any time you have a disk that is smaller in size paired with disks that are spinning at 7200 rpms, the smaller disk will vibrate because the larger disks on the right can take the vibration and it shakes the physically smaller disk because the harmonics from the disks on the left are shaking the entire enclosure. To correct this I put some bolts I had laying around on the smaller disk as weights to keep the smaller disk from shaking.