When I turn on my fan to the highest setting with either the heat or AC, it is very loud (I can hear it from outside the car with the doors and windows closed). It seems to be much louder with the AC than the heat, but is definitely very loud with both. I don't think it moves quite as much air as it used to either. Any ideas?

Ive had to replace fans in a few of the cars ive owned,...mostly domestic though. Usually its been a moisture issue getting into the fan area, causing rust to form on the spindle, even the electric motor's inners....this causes it to squeel something FEROCIOUS!

What year is your 4Runner? Maybe some blockage in rainwater draining areas? (that was an issue in an oldsmobile we had) If you do decide to talk to someone about replacement, dont be hesitant to have a face-2-face with a Toyota service person. They tend to want to get off the phone with you ASAP, but a face-2-face always works for me.