Heya guys (& girls???

)
My mums car has got some problems lately, and I cannot figure out what it is.
There is a loud noise coming from the car (as if the exhaust has a big hole in it / or as if you're driving a 8 cyl in a fiesta).
I really cannot figure out where the noise is coming from, but I do know this:
You can hear the noise when you go faster then 40kmh (about 25mph), it has nothing to do with the gear it's in, nor the brakes...
I turned off the engine while driving about 50kmh (30mph), and the noise remained (so it's not coming from the engine)
What can it be?
the car's a Ford Fiesta 1.1

from '89, dunno how many k's its got on the clock, because the clock didnt work for about 3 years (and suddenly it did work again

)
Cheers!