Well, there's two mechanical parts in a TiVo:
1. The hard disk, and
2. The rear cooling fan
Next time it happens see if you can narrow it down by listening close. The fan don't spin at a huge rate of knots so a simplistic test if you don't/can't pull the cover off would be to insert a drinking straw through the rear panel fan cutouts to temporarily stall it. If that stops the noise then there's your problem.
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