Completely unscrew the tension knob, pull off the little spring, and you will find a block - pull that out (or tilt your wheel so it falls out) and you will find an an arch, or a ‘U’ that sits on the shaft. This creates the tension. The ‘U’ is lined with a bit of felt. That felt bit gets smooth and can start squealing. Replace the felt with wool felt (not acrylic) or leather or suede.