I was in a similar situation as actusreus. I was having the swooshing noise with my 600rpm motor as well. I decided to go ahead and replace my motor. I made the switch to the 300rpm motor and large pulley. It has made a significant difference in lowering my noise floor. The motors are the same price and I would advise switching to the 300rpm motor.
Actusreus, I think you and I exchanged messages about the lubrication of the VPI motor awhile ago in a different post. It could be useful if you can locate it. I did quote all information I had from mike in that particular thread. Mike said to start with one drop of 40wt mobil 1 synthetic under the brass collar. My problem was I still had the swooshing noise and decided that if one drop didn't solve the problem then another drop might help. This wasn't the case. Too much oil will cause the swooshing noise to be more pronounced. I also think dust can become an issue over time and cause the sound you speak of as well. Replacing the motor was the only solution. The process to switch the motor is straight forward and simple. All wires are connected with wire nuts and no soldering is involved. Anyone with slight mechanical skills can do this without a problem. If you have enough skill to correctly setup your cartridge then, I would venture to say you will have no issues swaping the motor as well.
Actusreus, I think you and I exchanged messages about the lubrication of the VPI motor awhile ago in a different post. It could be useful if you can locate it. I did quote all information I had from mike in that particular thread. Mike said to start with one drop of 40wt mobil 1 synthetic under the brass collar. My problem was I still had the swooshing noise and decided that if one drop didn't solve the problem then another drop might help. This wasn't the case. Too much oil will cause the swooshing noise to be more pronounced. I also think dust can become an issue over time and cause the sound you speak of as well. Replacing the motor was the only solution. The process to switch the motor is straight forward and simple. All wires are connected with wire nuts and no soldering is involved. Anyone with slight mechanical skills can do this without a problem. If you have enough skill to correctly setup your cartridge then, I would venture to say you will have no issues swaping the motor as well.