Out of nowhere, my 2412 arbor began making a tapping noise that varies with the speed of the arbor (almost sounds like a car engine valve tap). Fast arbor speed = rapid tapping (almost a hum), low arbor speed = tapping which slows in rapidity as the arbor slows to a stop. I removed the blade and this had no effect on the noise. I disconnected the drive belt and, surprise, no noise, so it appears to be originating in the arbor. Anyone have any similar experiences? If it is the arbor, how difficult is replacement?