What could cause a loud metal scraping noise in the back of lincoln navigaitor while backing up?

It’s difficult to say because engines make a wide variety of noises.
Rattling, possibly a worn timing chain.
Misfiring, possibly incorrect valve timing or damaged head gasket.
Metallic scraping, possibly a lack of oil pressure causing bearing to run dry.
Knocking, possibly worn mains or big end bearings.
Loud exhaust noise, possibly a blown exhaust manifold gasket.

There are a wide variety of possible noises and therefore a wide variety of possible causes

If they're drums could be rust on the drum rubbing on the backing plate, or a loose brake component rubbing inside the drum.

IF EVERYTHING WAS OK THE CAR WOULD NOT SQUEEL!!HUMIDITY CAUSES SUCH NOISES!!LET ME KNOW

Make sure its attached tight, take it to a local repair shop if that does not help.

Try to better describe the noise.
Is it a
scraping of metal
grinding / rubbing
high pitched shriek like a drive belt slipping
Is it
underneath the body
under the hood
in the aircon
when you pull into gear
when you pull away
etc
Do a bit of detective work and come back.

what type of noise is it a scraping noise if so need new shoes on breakes asap befor down to metal causing damage to drums or try scraping a big screwdriver around edge as a build up of rust around edge can also cause noises

have the breaks checked....make sure the lugs are all present and do not appear loose....see if the brake fluid is low too...if that is what's leaking....

The scraping noise may be a "tire rub," where one or both front tires are rubbing the wheel
wells during a turn (going forward or backing up).
Cause: possible ties size may be a little larger than the tires specified by Dodge for the vehicle.
Check: the driver's door panel, which will show manufacturer-recommended tire sizes and inflation pressures.

YOU BETTER GET TO A BRAKE SHOP AND FAST, SOUNDS LIKE YOUR REAR BEARINGS OR TIE RODS ARE GONE AND YOUR PADS ARE WORN DONE SO BADLY THERE IS METAL TO METAL CONTACT. YOU BACK WHEEL COULD FALL OFF. VERY, VERY DANGEROUS.

You could be right with the wheel bearings. But first take the wheel off and check that nothing is scraping the rotor. Look all the way around the rotor. A suggestion will be that the brake pads have worn enough that the brake pad wear indicator is starting to scrape. Pry them back a little.