Remove the BB and wipe down the grease. Check the grease for any particles. Try using a clean ball bearing in the cup and use your finger to roll it around and see if you can feel any irregularity. Look at the bearings carefully. Using a ballpoint pen run the tip aroung the spindle carefully and held lightly. It may pickup a very slight irregularity. Your fingernails aren't very sensitive. What you describe sounds like a very slight irregularity.
Was the BB faced properly with no residual paint on the faces. Good luck
I wouldn"t run less than the proper number of Balls.