Without more details it is hard to say for sure, but in my experience, this is usually one of two things.
Either the audio interface settings are insufficient for the computer to play sound smoothly,
or the sample rate of the system and the Logic project are not the same.
If the audio settings aren't giving the computer enough time torender your plugins and mix data you may get pops and crackles as it drops the audio for a few milliseconds here and there. Usually you will get an audio drop out notification if it is quite severe.
But it is still possible to get the system into a state where it's good enough for Logic to play back, but not good enough for human consumption!
The sample rate issue is usually something along the lines of a project set for 48k sample rate and the system is set at 44.1k.
This is close enough that the system will make a good effort to playback the data and it works for the most part. But then you will get intermittent audio artifacts. It's kind of like the audio equivalent of a Moire pattern. For some reason this problem sounds very different on sound cards from different manufacturers. On my UAD system its almost imperceptible. Just a slight kind of clicking sound that is very quiet. On my MOTU M4 it's much more aggressive, like a pencil poking a speaker.
I've certainly heard it sound like static.