What do you think causes the crackle and clicks we hear?

I think pops are easier to understand but clicks and crackle noises, what causes this?
If you play an early pressed well cut platter the noise is minimized. For mo so most records available for sale the only way to minimize noise real time is an expander, Park's Magic or spend up for a Sugar Cube.
dirt. I have found DCC, Analogue Productions, Umbrella D2D and Sheffield Lab D2D pressings are VERY quiet. ANY record I purchase - new or used - gets cleaned in my CleanerVinyl RCM. Some of them multiple times. 
Zero Stat lol.  Unfortunately, I see absolutely no improvement when using.  I'd say best used as a paperweight, only its not heavy enough.  The maple shade static draining brush looks intriguing.... cheaper too!
I have the same issues with these old records I am cleaning using the SpinClean record washer.  Most show a lot of improvement in amount of noise, but there are certain records that continue to pop and crackle.  I clean them a second time and not much improvement.  I was chalking it up to excessive grove wear — I know that the equipment they were played on decades ago was not very good; neither was the care and cleaning (some were my teen party records).  I hadn’t thought about static electricity.  I will need to look into that.

On another thread, I asked for advice for reducing the warp in one record that I have. I think it was affected by heat somehow.  My idea was to use a clothes iron to heat the record and press it flat.  I laid the record on a paper record sleeve on a flat surface, then placed a microfiber towel over the record and then applied the iron (no steam).  I went slowly and started out with low heat.  The warp decreased but not enough to play it.  So, I gradually upped the heat and it was still hard to get the record flat — If anything, it warped more on other parts of the record.  I think it would take a special device that could apply heat and pressure equally over the entire record surface all at once.  Otherwise, it just continues to warp in a different way than when you started.  I was looking forward to hearing those island rhythms too!
The Zerostat has always been a pain to use, and only marginally effective. I replaced it with a Nagaoka Kilavolt No. 103 (no longer in production) years ago, and now also have a Furutech Destat III. Both are much better than the 'stat.