whooo sound like a loud owl under driver seat

[sukkafu]

i have a 1999 sho with 47k on it. i have a loud whoo sound when the car slows down or when it is braked. i had my mechanic buddy check it out. first i had new brakes. then the noise continued. it wasn't coming from the brakes. then my mech said it could be the catalytic converter. my dealer is not very trustworthy and the 3rd day after buying this new in sept 99 off the showrroom i had to have the tranny fixed ! i love the car and the appointments and the sound system. my car is 100 percent stock. i have never gotten more than 20 mpg on the freeway and 16 mpg around town. the car shimmys left and right when i slow down- must be alignment or wheel balance?
i am not a very astute car person. i only want to see if more expert folks like yourselves could tip me off to some answers. the loud whoo sound comes even when coasting to a slow down or stop.

 

[SHODUDE]

Does the 99 have an sir charged back support in the seat? If so, thats exactly what it is.

If you put too much pressure in the seat, the bag will overload and the pressure regulator releases causing a sound identical to what you are describing.

Try letting all the pressure out of the seat and see if you still hear it.