As others have rightly said, the problem you are experiencing is called clipping. Clipping occurs when the signal input to a step of sound processing (such as the mic capturing the sound, the computer recording it, or any hardware in between touching it) can't handle the level of the signal in whatever form it is in. The end result of clipping is that the maximum value is applied (or all values stop being applied in the case of a dynamic mic).
This results in a break in the waveform which can't be produced by a speaker and lost information that the system was unable to capture, which is why you hear a grainy and distorted sound. Sometimes this can be considered a good thing, such as overdrive on a guitar, but most of the time it is not, such as your current experience.
Again as others have mentioned, to fix this you need to reduce your signal level. The easiest way to do this is to back off from the microphone so that it isn't driven beyond it's limit. You can also try turning down the volume of the recording or the amount of pre-amplification done if your mic has a gain adjustment to try to reduce the signal level while still close to the mic.
If you are simply producing too loud of a sound for the mic and you can't move back from it, then you will need a microphone that better handles loud sounds. In general, condensor mics, such as your AT2020 are designed to be highly sensitive, but not to deal with very loud sounds. A dynamic mic is much more able to deal with such sounds, but you would need an audio interface to work with most of them.
So, in summary, you need to find where the signal is clipping. If it is in the computer, turn down gains and recording levels to bring things within range. If it is in the microphone, step back or switch to a mic capable of handling the intense sound levels.