What is the procedure to unobstruct a blower and defrost fan in a 2000 Mitsubishi Mirage?

You will have to take loose the duct work for the fan and run a vacume cleaner hose up in it clean out the obstruction if you can reach it with some other object like a straightend coat hanger , its just one of those things you have to work with till you get it , tring different things

If your heater is makiing a noise like a flapping sound it may be just a leaf or piece of paper that found its way into the air duct assembly. This also can happen if something falls down into the windshield defroster duct. This can make a racket but most times it is not harmful and eventually may work its way out.
If it is making a rumbling sound then it could possibly be a fan bearing going bad which is not uncommon..
If you have an air flow problem where the air is not coming out with force then the possibly that a heater door not opening or partially open.
I don't advise poking anything up in the duct assembly it is possible to make the situation worse than it is.
It is not easy to access blower areas in most cases as they have to be sealed tight to deliver proper air flow.
A heater a/c specialist may be a safer option.