When I'm Driving I Hear The Sound Of A Helicopter Or Jet I Only Hear It If I Have The Back Window Down And I Can Not Figure Out Where It Is Coming From I Need To Know Do I Have A Big Problem Or Is It A Small Issue That Can Be Fixed And Not Cost Alot!!!!!!!!!!!!

Check the luggage rack or rear dust/bug deflector.
May be loose or broken or falling off.
MT-2500

No, this happens if you only have one or both of the back windows down, the air makes the roof vibrate

You might run it by a body shop and have them look it over.
Mt-2500

It's normal for a blazer.

Its normal for almost any SUV, it does it in a Tahoe, a Durango, etc. Just roll one of the front windows down a bit.

It's normal for a blazer.

It might be youf cooling fan clutch, or......

Stay off the RunWay