Try using two O-rings on the yellow stir bar. With only a single O-ring, it will always be out of balance and likely be noisy. Stir bar noise is typically due to one or more of the following:
1. Poor magnet alignment on the fan face. (must line up with center line of fan)
2. Poor magnet balance on the fan face. (must be equally spaced from center of fan)
3. Wrong magnet spacing for a particular stir bar.
When using a single bar magnet, No. 2 would not apply as the spacing (of the magnetic poles) would be fixed. A properly aligned and balanced stir plate with the correct size stir bar should run silently or very nearly so.