Whistling sound when shifting gears
 

Today I noticed the car is making a whistling sound (sounds like air swooshing not mechanical) just before it shift from 3rd to 4th gear.
Otherwise car is behaving normally in all gears, idle, acceleration, etc.

Any ideas where to look for the problem?

Does it sound kinda like a train horn?

similar note but faint. Like a train a mile away.
You know what it is?

Nope, but i've noticed it (what i think you're describing) too sometimes. But barely noticeable... so long as transmission is behaving normally, i think its fine :)

How cold is it in Canada these days? Maybe that has something to do with it. Does it vary with operating temperature? Have you been parking inside so things can melt off the car?
I'd do a quick check for vacuum leaks just for fun.

Well Ontario is not any colder than Michigan.
It was parked inside to repair the disa flap so nothing would've been frozen. Drove it over the weekend and didn't notice any more whistling noises when shifting.
But, disa has been repaired, performance is normal, cel is still on. I was hoping that after 5-6 cycles the cel should reset by itself. Am I wrong to assume that?