Clicking noise from battery fan

Hi everyone. Whenever the battery fan is running near or at full speed (e.g. ac cranked up high, or on the highway), I can hear this strange clicking noise, as if something is caught in the fan. Mind you, the sound isn't there when the fan spins at a normal speed, only when it's working hard.

I'll also post a sample shortly. Could it be some debris trapped in the fan, or possibly the fan giving out? For the record, I have cleaned the grille of the fan, but that's about it (didn't solve the problem, obviously).