As for the fan, some fans on servers these days are thermostatically controlled, and so don’t run all the time. That could well account for the intermittent portion. As bonvivant pointed out, fan replacement is pretty cheap. Other sources of intermittent noise are the cpu fan (if not ducted from a larger one), hard drives, and CD/DVD drives.

However, if you have several fans in the system (typical with server grade equipment), I’d recommend that you go by an auto parts store and buy a mechanic’s stethoscope, and use it to isolate which fan is actually making the noise.

Just make sure that whatever replacement fan you buy is ball bearing or better yet double ball-bearing. The price difference is only a few dollars which is nothing compared to the downtime for replacing it cost-wise.

Bob