This humming noise can be easily repaired by replacing the idler tire. The idler tire is typically located on the underside of the cassette player. You can access it by removing the cover of the cassette player. Once you have removed the cover, you will need to locate the idler tire. It is usually a small, black rubber wheel.
To remove the idler tire, you will need to use a small screwdriver to pry it off the spindle. Be careful not to damage the idler tire when you are removing it. Once you have removed the idler tire, you will need to replace it with a new one.
New idler tires can be purchased online or at electronics stores. When you are replacing the idler tire, be sure to install it in the correct direction. The arrow on the tire should be pointing in the direction of the tape movement.
Once you have replaced the idler tire, you will need to reattach the cover of the cassette player. You can then test the cassette player to make sure that the humming noise is gone.