Snap, Crackle, Pop - What is the cause?

First, I love vinyl. I would never say it is superior to digital but it truly sounds different. I am able to make an emotional connection to the music from an LP far more than from a CD.

That being said, I buy a lot of used vinyl at records shows, shops, etc because there really is so much available and for the most part, cheap too. The only thing that always intrigues me is the snap, crackle, pop on some records and not on others. Even they are the same title, label and pressing. Some of it is so bad,, it make an otherwise well recorded album unlistenable especially on soft passages in classical music.

So where does it come from? Is it static? Is it worn, damaged vinyl from being played on cheap and less than desirable cartridges from back in the day or is deeply ground in dust and dirt embedded in the grooves?

I clean all my recently purchased used vinyl with a Spin Clean system first and bag them in anti-static MoFi sleeves. I still get the Rice Crispies on some receords but not all. I buy and enjoy enough of it to consider investing in a better record cleaning machine like a VPI or Okki Nokki, but before I do I really want to know where the Rice Crispies on some records come from. If it is damage from being played on cheap equipment, no expensive machine will help. But if its dust and dirt, then maybe I should get an vacuum cleaning system and will take care of it? Your guidance is greatly appreciated.