I have noticed (probably 4 times since I bought my nissan quest in 2007) that for a split second the blower runs at full speed and then comes back to normal. like it's sighing . I think at all instances the AC was on. does it detect a need to circulate a whiff of air?
Climate control? Don't lose sleep over it. My Benz air-con behaves exactly the same way at times, and Auto Hangar insists that's the way the blower is supposed to function.