Lately I've been noticing a rattling noise coming from my SB-220's cooling fan upon cold start. It's not a ticking, but a definite rattle which is quite unnerving. The weird thing is, it goes away after about 30 seconds. I'm guessing a bearing, sleeve, or bushing which houses the fan shaft warms up and via thermal expansion, whatever play goes away. It doesn't do it all the time and the fan generally remains quiet for hours except for the hum of the blades spinning. I haven't had my amp apart in a while. I can't recall if this unit had sealed bearings or one with oiler holes for the bearings. Either way, should I take this as a sign that it's time to replace the entire motor assembly? If so, anyone have any experience with the direct replacement units Harbach sells?