All my gear driven machines whine somewhat. I have a 15-91, 2 401's, a 500 and a 503. They all have their own "song" that is different from belt driven machines. Does it sound a lot different than it did before you greased it? It shouldn't hurt to grease the gears (be really careful not to over grease the gear on the top of the motor shaft though). Maybe it is whining more because it is going faster now!
Pat