2011 Ford Flex Whooping sound

check the drive line and "U" joints to the rear diff. probably not installed correctly and out of alignment

I would guess an Air filter needs to be changed. Or there is a hole somewhere in the air delivery system.

CHEAP TYRES RESONATE AT CERTAIN SPEEDS

check for slipping ac drive belt ( smell and noise )

I'm at work right now, but will help you out as soon as possible.

Noise might me coming from breaks if pads are completely worn down.

sounds like front wheel bearings. does the noise change, get loader when making turns?

Wheel bearing !!

I would take it back to the place U got it from and tell them U have a CV joint problem and take care of it. I would hope this is covered under warrenty.

These used car dealerships are bad. We have had same issues with getting a used vehicle and it having problems but the dealership takes many times to fix the problem.

Hope this help U

Matt

if you have a constant whine that raises pitch the faster you go, you have improper backlash adjustment in the differential (rear end), if you have a whooping noise that whoops faster as you drive faster then you have a bad wheel bearing, both can be the result of low fluid level in the rear end.