I'm surprised no one has responded to this one... It's been a few months, but hopefully this will be helpful.
I've had this issue on a couple vehicles... and it typically has to do with debris getting sucked into the outside air intake, and falling down into the duct. Leaves, sticks, a small scrap of paper, etc. Sometimes this will get lodged between the fan and the duct housing, so as the fan turns it's hitting the debris and you get this ticking sound. If you park your car frequently under trees that shed, this is pretty common. Many newer cars now have air intake filters that help eliminate this problem, but the vast majority of cars out there don't. The other possibility is that the fan itself is out of alignment or out of balance... but it's more likely that you just have gunk in your ducts. To fix this I always had to remove the fan/motor assembly, then suck out the debris with a shop-vac. I suppose you could try sucking it out the top of the intake without removing the motor, but I've never had any luck with that method. Best of luck.
