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So I've only had this happen once for a matter of seconds, but I worry it will worsen with time.

I was driving on the freeway, on a straight, non-hilly section, when my front passenger-side closed window started making an ear-splitting squealing noise out of hell. As far as I know, the window was completely shut. I opened it for a few seconds (no squealing during the movement at all), closed it, and it stopped.

Any ideas on what could have caused that and fix-it solutions?