Zalman NC2000-strange noise coming from the fans

I have been using the nc2000 for a year or so, it works great so far, but recently when the fans spin on max speed, there are some annoying noise coming from fans.
I thought it came from my internal laptop fans first, but later I found its the cooler.
It doesn't happen all the time, like the noise will keep on going like 5 min then it will disappear, is this a sign the cooler is failing? I cleaned the fan blades with compressed air, but it couldn't remove all dust clings to the blades, the noise still happens.
Any idea why does it happen? I do turn on the cooler most of the time, mostly on max speed, will it reduce its life span? I would hate to shell out 60$ for another quality cooler if it failed.

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