Does it happen only under load, or does it happen when it is stationary and you rev it?
Also, is a very loud noise or just a faint one?
One thing that i know that makes a very noticable whirring noise, without hearing it, it sounds like something running off your auxillary belt has dropped a bearing, and the most common for that is the power steering pump. If your power steering pump is dieing it sounds like a cross to a straight cut gearset whine and a supercharger drivebelt whine.
The best way to figure this out properly, would be to take it into a decent service workshop and they should have a stethoscope, similar to what doctors use but it has a steel probe on the end that you can put on different mechanical components to figure out where the origin of a noise is. They should do this free of charge as it is a simple task that takes less than 5 minutes.