We actually had a printer service call at the library last week! We have a very old HP monster, non-color, with a duplexer and add'l feed tray at the front desk that is mainly used for printing book spine labels. For some reason the lower feed tray was making a horrible clacking grinding noise. The tech's thoughts was that there's a sensor next to the paper out sensor that lets the printer know when it has elevated the feed tray fully, and that was not working right.

I had the printer working by simply removing the paper tray. He ended up fixing it by replacing it: apparently an El Paso, TX company ordered like a dozen of the feed trays in yellow, then never took delivery, so they had spares.

It's interesting. The company is in Las Cruces, about an hour away, this guy is based in our city of Alamogordo and basically does the county region, and their warehouse is in El Paso, Texas, about 2 hours south of Alamogordo.