MacBook Storage Fluctuating Rapidly

My first guess would be some cloud based file syncing issue, or file sharing issue such as using BitTorrent.

Try booting into Safe Mode to see if that makes any difference. If this works, then most likely you have an issue with some third party software although certain Apple items could be impacted as well.

You can also turn off WiFi to confirm some app/service is downloading stuff onto your drive. Both of these options may give you a reprieve in order to better investigate the source of the problem while also providing a possible clue.

Could your app you are developing be accidentally causing the problem?

You can also try creating a new macOS user account. Log out of your main user, then log into the new user account. Does this fix the problem? If so, then the issue has been narrowed down to your main user account (perhaps a Login Item, or a configuration issue, or a cloud based issue).

From another contributor @etresoft regarding Free Space and Available Space 

For context >> Free vs available disk space huge differe… - Apple Community

Quote >>  “ The "available" storage is the amount of used storage that the operating system could automatically delete if it felt that it was really necessary. The "free" storage is the amount that you can actually use for something.

There are system processes that run in the background and automatically delete some of the "available" storage and convert it to "free". If you completely run out of storage, then those system processes will try a little harder. When you "delete" files you are just hinting to the operating system that you don't need those files anymore. The operating system will eventually remove them, but on its own schedule.

Certain tools will allow you to force the issue and manually clean up some of this storage and manually delete local snapshots. But that is only temporary. "  << End Quote 

OP wrote " or a virus. "

There are NONE, " viruses " - Full stop

Sorry but this shows a general lack of knowledge regarding macOS

So one is better to look else if you believe a Virus is the root cause.

Windows-like viruses that self-replicate and affect macOS don’t exist because of the underlying UNIX Foundation and permission limitations.

The macOS operating system resides in a sealed and read-only volume that can’t be opened by users or third-party applications.

Mac app security enhancements are sufficient to protect computers from malware.

Adware and malware can affect computers and are often downloaded from “shady” websites or developers who inject them into downloads.

Source: User tip from @Kurt Lang

What is malware? - Apple Community

Think we have spend enough time on this question and best ti leave this to the user to figure out

Good Luck

JWHero123 wrote:

Well then I guess I got mixed up. Regardless, the issue self resolved, it's not fluctuating anymore, so Safe Mode now tells me as much information as a logged in user does. ........ The only way I could access it was through Safe Mode, deleted some files via Terminal so the OS had some breathing room after rebooting, and eventually it fixed itself.

I'm glad Safe Mode helped a bit. I guess whatever app/process that had work to do either got finished or maybe Safe Mode helped to disrupt it.

I don't know if MacOS reserves some space to prevent from rendering the computer unusuable for basic operating system procedures,

Unfortunately that concept seems to be unknown to Apple. Linux has a similar type file system (BTRFS) and they implemented a reserved buffer to prevent the file system from having this exact problem. At least with the Linux file system, before they created the reserved buffer area it had the ability to add an extra storage such as a small USB stick that allowed me to free up some space on the main volume. You can provide Apple with product feedback here:

Feedback - macOS - Apple

You might be getting wrapped up in that my filesystem said it had 35 GB of purgeable space and Finder counted it as available space. This did not happen, I had 45 GB of space on my SSD ready to use, something consumed it, and then freed it. None of this BS purgeable space you're trying to get off topic with.

I wrote my second post because you mentioned and I quote for the second time:

Phew, I used the correct term "free space" because I determined it by Finder > Macintosh HD > Get Info. I was referring to free space here determined by the filesystem within finder, not the available space for the partition determined in Disk Utility

Which gave me the impression that you did not understand my original explanation of how to know the actual Free storage space. While the "Available" value in your case only included a small amount of Purgeable storage, that is not always the case which can make a huge difference....you just got lucky in that one instance. It was meant for educational purposes because any mention of Available storage is too misleading to use unless the Purgeable space is also mentioned so the Free space can actually be calculated. Plus keep in mind that I need to consider other people who will read this post, so I need to make certain that what is discussed is clear....if it wasn't clear to you, then it is likely unclear to others as well.

Follow on but the topic is different that yours

The Issue of Used versus Free space is the intent and focus as to why you are alerted to this

Specifically the posting by my good colleague @HWTech who also appears in your question

Software update suddenly calling for twic… - Apple Community

Disk Utilities will give you a much more accurate tally of Used Space versus Free Space

Finder >> HD / Get Info ?

Not so much so

JWHero123 wrote:

Phew, I used the correct term "free space" because I determined it by Finder > Macintosh HD > Get Info. I was referring to free space here determined by the filesystem within finder, not the available space for the partition determined in Disk Utility

The information the Finder reports to the user is using the "Available" storage value which is very misleading. The Finder reports the Available value and in parenthesis the Purgeable space. You can use them to calculate the actual real Free space value:

Free space = Available space - Purgeable space

Available space = Purgeable space + Free space

Purgeable space is storage that will be released by macOS at some unknown time in the future, so the Purgeable space is cannot be used immediately. Only the Free space can be used immediately. The Free space value shown in Disk Utility is the most important & accurate storage value there is in macOS. Even the Used space value can be misleading if you make copies of data within the same APFS volume due to how the APFS file system works and how macOS is reporting this information.

Did you try Safe Mode or another macOS user account to compare results as I suggested in my previous post? These two troubleshooting tips may be able to further narrow down the source of the problem.

I have also experienced issue with fluctuating disk space. Main task was kernal task. Was freeing up loads of space and then seemed to be consuming it as soon as I freed it up.

Checked there were no updates as automatic updates are on and this can trigger this type of behaviour. No updates last checked today.

After restart it went up to 10GB then down to 3 back up to 5 and now on 11 within a coupe of minutes?!