An audio (sound) signal normally has a nice rounded top and bottom. If the top and/or bottom is flattened off, it is said to be clipped (like the bottom of a flat tire). Under most circumstances, clipping occurs when the input from your instrument or effects device is to high for the amplifier. The resulting amplifier output is distorted and sometimes injurious to your speakers. On the other hand, when a clipped signal is introduced at a low level you will get a distorted sound referred to as "fuzz." Fuzz boxes were all the rage in the early '60s and some of the "effects machines" still produce a "fuzz" tone along with many other sounds. To hear the original "fuzz" sound used in a recording, get a copy of an old Marty Robbins recording of "Don't Worry." At the beginning of the second verse you will hear the "fuzz" introduced accidently by a bad tube in the recording amplifier.
They thought it sounded cool and left it in.
Hope this helps.
BigDon
For popping and crackling, the first thing I would check is cables. Try another cable and see if it helps. Twist your controls and see if it gets worse or better. If the controls are dirty they can cause a problem and turning them back and forth will sometimes fix the problem. Otherwise it could be an internal component going bad and you will need to take it to a qualified technician.
Good luck.
BigDon