SPCR

What is it called when a fan making an undulating noise again? Is it resonating?

Anyway, this is happening with a HP ProLiant MicroServer, which has a single 120mm case fan (and a PSU fan I think).

What can I do about this?

Thanks.

Could you describe the problem better or post a recording? Or at least tell what the period of this undulation is.

If you have an abnormal noise problem, take it up with HP and they should fix it for you. The best way to demonstrate you have an abnormal problem is to compare. So a recording of the noise would be very convenient to establish any abnormality.