AM or FM reception ?

Is this a factory or aftermarket radio.

Static gets into a radio via the inputs and outputs of the radio as well as
if the radio is not healthy internally.

That means antenna mount and lead, power leads into the radio, speaker
leads, and grounds.
Failure of shielding or other noise reducing circuits causes problems around
noise sources

I'd start with making sure the antenna is in good shape, if a fixed external
fender mount model look for crud where the antenna screws into the fender
mount.

Get a walkman type radio, with the vehicle running and the walkman tuned to
the stations with problems, carefully and safely wave the radio around the
engine bay and other areas until you find the source of the noise..
Becareful of moving parts. headphone wires don't like drive belts, etc.

Replace the alternator.  Brushes are wearing out.

Rob

My experience over the last 20 years with Fords has been
that the electrical system becomes noisy over time.  The
things that help are making sure all the grounds are good.
The in-tank fuel pumps seem to be a source of RF
interference that can be helped with various choke schemes
that can be found on the internet.  Some of them have a
device that can be installed in the pump wiring inside the
tank to stop the RF problem.  Sorry, I no longer have the
TSB with the part numbers.  The thing that has corrected the
problem for me every time has been a choke installed in the
powere supply at the back of the radio just like we've done
it for the 50 years I've been working around vehicles with
AM radio problems.  The one I have been most successful with
looks like a small cannister.  Radio Shack used to carry
them but, their clerks now hardly know their way to the
shop.  I have bought hte last couple at Fry's Electronics.
You can find them on the net with google for $3-10 each.
All you need is one.  The one I have found that works well
looks like this one.  They are avaialable to accommodate the
line load thru them.

Lugnut

http://radioproshop.com/filters/ns12.htm