As Kwil says a slight twist in the arm can cause this, the evidence is that it makes the noise in one direction.
If you watch the blade as it sweeps the screen you should see it flop over equally as it returns from the end of its travel, if it doesn't you will have to tweak the arm carefully, testing it after each adjustment on a wet screen.
As others have mentioned nothing will be lost by giving the screen a good clean by the methods advised.
Ref. RainX, it is excellent stuff, I use it on all the glass except the windscreen, I used it on the windscreen once and at speed the raindrops beaded and blew up the windscreen giving a clear screen, however at lower speeds I found the beads were a nuisance, more so than on an untreated screen, especially at night and on intermittent wipe.
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