"dave" nospam wrote in
:

Should I be concerned? What is this fan's job? Is it to keep the
oven ventilated of food vapors or to cool the magnetron? If the
latter then I need to replace it asap.

From the FAQ:
Since 30 to 50 percent of the power into a microwave oven is dissipated
as heat in the Magnetron, cooling is extremely important. Always inspect
the cooling fan/motor for dust and dirt and lubricate if necessary. A
couple of drops of electric motor oil or 3-in-One will go a long way. If
there are any belts, inspect for deterioration and replace if necessary.

http://repairfaq.cis.upenn.edu/sam/micfaq.htm#miccfans