I have a 2006 Mazda 3, 88km's.
Recently over the last couple of weeks I have noticed a small whirring sound coming from inside the AC vents only when I am decelerating and coming to a stop at the lights. The strange thing is this doesn't happen all the time, it starts only after about 10-15 minutes of driving and under normal conditions of driving under power there is no noise whatsoever. Here is a complete list of symptoms and tests performed:

1) Only starts to make noise after 10-15 minutes of driving
2) Only happens when car is slowing down (i.e. accelerator off)
3) Doesn't happen all the time, maybe 30-40% of the time
4) Doesn't happen when AC is turned off and standard blower enabled
5) Temperature makes no difference although I regularly keep mine at 24'C (living in Singapore)
6) When sitting idle at lights, revving the motor makes no difference.
7) Changing from Outside Air to Recirculated Air makes no difference.
8) Tried maximising the fan speed to blow any fouled articles, no difference (Car rarely gets leaves on it as it is stored in garage.
9) Actual air temperatures are still running fine through all vents, coolent was done a few months ago.

I have read through dozens of forums on this subject and so far although a couple of people have had similiar issues, as usual the final solution is never posted. The sound is that of cards hitting the spokes of a wheel. It seems to happen when whatever fan unit behind the dashbaord used to control the AC is spinning down to a stop. Obviously this unit is tied to the motor somehow and is spun up when car is accelerating.

Any ideas? I'd rather get some input before going to a mechanic.

 

After much further online research I believe the problem is possibly the AC blower motor under the dash may have some twigs/leaves or even the fans may have a problem. Therefore I am going to book it in for a service and ask them to check it out. I have seen the hassle of trying to do this yourself on a 2006 Mazda 3 from online YouTube video's so its best to be done by a authorized mechanic. Will get the air cabin filters changed at the same time as I beleive they may need a change. Will update later this week on progress.

 

Further update. Took the car to Mazda dealership and had technician take the car for test drive to hear noise. He quickly told me it was the AC compressor not the blower motor. At the end of the drive he kept the engine running and popped the hood and we could both clearly hear the "unusual" spinning noise of the compressor when it clicked on. The parts alone are going to cost me SGD$1470 (USD1132) exluding a couple of hours labour.
I guess i just picked up on it early enough as the technician mentioned they can get really loud after some time.

 

glad you got it sorted, sorry to hear about the price