2002 Tahoe squeaking noise from passenger side fan?

I have a 2002 Tahoe with 80,000 miles. A few weeks ago I noticed a squeaking noise which seems to be coming from the passenger side air vents. When I turn my climate control fan off the squeak stops but resumes when I turn the fan back on. When I have the fan on low the squeak is quieter but the higher I set it the noiser it gets. Any ideas on what this might be?