There are three main possibilities that are on my mind for what your problem is:
1) The blower motor is going bad and just needs replacing. I'm not sure where the access to the blower motor is on your vehicle, but generally this is a fairly easy repair to do.
2) The blend door in the HVAC enclosure is sticking or the motor is being overdriven, producing a repeated "clacking" noise. This can often be remedied by turning the temperature control all the way to hot, waiting a few seconds for the sound to stop, and then turning the temperature back down to the desired cooling. Another possibility is to pull the fuse for this circuit, turn the ignition to on, wait a couple minutes, turn the ignition off, replace the fuse, and turn the ignition on. This may reset the circuit memory and fix the issue.
3) Animals. The blower motor housing on cars is a common refuge for rodent folk. You can usually tell if they've taken up living there from the smell though.
I would try the blend door first and if the sound doesn't go away from adjusting the temp to max on both ends of the spectrum, pull the blower motor and see if anything got inside the housing, or if the motor is going bad.