Is the AC on? The fans should turn on any time the AC compressor engages, and they'll turn off as soon as the compressor disengages, unless a temperature sensor tells them to stay on. My guess is your AC compressor is engaging and disengaging every 20 seconds and that's making the fans turn on and off.
I'm interested in this, I have an '07 RAV4 that I've been noticing this same thing with the a/c on. I'd like to know if this is normal or not. A year or so ago I noticed the a/c wasn't blowing very cold, I recharged it with some of that bottled stuff and the gauge on it seemed way off given the amount of time I had it on there. I was afraid to over charge it so I took it off after awhile. The a/c blew cold again but I've noticed it cycles on and off like described above, but I don't recall if it always did that or is a sign of being over or under charged.
Not sure if it's applicable, just something to be aware of and possibly an answer, but because the A/C is also a form of dry air, it's commonly part of the defroster mode on your car's heater. Is the car's heater set to defrost, or is the HVAC in some automatic defrost mode at startup?
