chattery maxi jets

[fishface]

so i've got 2 MJ600's that are so chattery but not really that old. i've tried swapping impellers around etc to see if i could get them to quiet down a bit but no luck. they are both however freshly cleaned and taken out of retirement from another application so does that mean they may just quiet down after a bit of slime-coat happens? or does anyone else have any idea how to keep them from being so loud?