That screech is caused by the wiper bearings.  You will need to replace 
them, or else the wipers will seize up and burn out your wiper motor.  I 
had this happen to my 74 in a huge rainstorm, so please take my advice.  
Fix it NOW, or you will be paying for a new wiper motor as well, which is 
very expensive.

The bearings are located at the base of the wiper arm, where they attach 
to the body.  There is a ton of linkage underneath that, which can be 
accessed from under the dash.  You might want to consider having it done 
for you, if you don't have experience with working under or removing the 
dash(I don't, and was charged only about 3/4 hour labor, around $40 I think, 
plus parts which I bought at a junkyard).

Craig

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Craig Standley				1974 Volkswagen Westfalia
University of Washington
Physics  				"Life is like riding the bus, it 
cstand@u.washington.edu			 requires change." -Dennis Miller
http://weber.u.washington.edu/~cstand    
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