When you let out on the clutch or push the go pedal down, you can experience a squealing sound as the drive belts begin to engage. And then it will stop once you disengage the drive system.
This is a pretty good indication that your drive pulleys are going bad. You can verify this by removing the lawn mower deck and locating the pulley and slowly spin them. If you feel any resistance or binding in the bearings, then you know that this pulley is bad and will need replaced.
The pulleys on most mowers today are made of plastic and the bearings are not of the highest quality. Dirt gets into the bearings and then wears them out quickly and they will need to be replaced. Replacing them can be fairly easy to fairly hard as sometimes the bolts go through the chassis and are hard to locate.
Make sure that when you replace the pulleys, you replace all of the plastic ones as a set because sooner or later, each one of them will wear out. And be sure that you have the belt installed correctly when you are finished. Taking a picture with your phone of the belt routing is a good idea to help keep everything aligned properly.