AC/heater fan clicking noise

I started to notice this problem not too long ago. When I turn my ac/heater fan on, I get a clicking noise that comes from under the hood around the front right passenger side of the engine. The clicking noise stays in time with the rpm of the engine. Once I turn the ac/heater fan off, the clicking noise is gone. There is usually a LOUD click once I turn the fan on, and then a LOUD click when I turn it off.