2002 Mitsubishi Eclipse Heater Noise

Heater problem
2002 Mitsubishi Eclipse 4 cyl 85,000 miles

Whenever I turn on my heat it makes a clicking nosie almost like the noise you would hear if you put a baseball card in the spokes of a bicycle. It just keeps clicking and wont' stop. It sounds like it is coming from the dash section of the passenger side. However, it doesn't seem to make the noise when my A/C is on and both the heat and A/C work fine. Could it be a motor or? Thanks in advance for your help.