From time to time (for instance, a few times last night at around 2am and now at around 11pm) my Time Capsule starts making the whirring noise it usually makes when backing but then the whirring stops and no back up is made (not that one is due). And this happens a few times in a row at 2 minute intervals. No problems are being reported in Airport Utility. I have restarted then hard reset the thing but no change. Any ideas? Is this usual?