I use SoundForge Audio Cleaning Lab to remove clicks and crackling.I check to see if stereo widening helps to move the distortion to a place less noticible. I then open the file in SOundForge Audio Studio and use a EQ and determine what frequencies are doing the most & least distortion, and adjust. Sometimes some frequencies can help cover up some distiortion. You could use your favorite audio tool, Audacity is pretty good and free.