squealing supercharger

my 95 previa supercharger makes a high pitch squeal or whine when you accelerate quickly ,when it is under load and as the van get up to speed 55 it quiets down .Iknow its not a drive belt or ilder pulley bearing
I have filled the resevoir 3 times this year with syringe & tube to factory spec s If its leaking i cant find it. its a sealed unit ! what can I do ? It is getting louder and more apparent , do I have to replace it or is there a rebuild kit