the surface tension of water or any liquid is much greater than that of air … therefore even if water doesn’t leak out it may draw air in. The feed side of the pump is only drawing vacuum so it wouldn’t force anything out but but the air is allowed in, and causes the surging/cavitation. I would try starting your machine and when it is surging like you say, try running a separate low pressure (like a regular garden hose) to flood over all the plumbing on the input side of the pump and when it comes across the air leak the water will suck in instead of the air (or at least stop the air) and you’ll notice a difference, that spot that makes the difference is where your air leak is. Could be just a pin hole or hairline crack and can be frustrating to find, but good luck 
Set your unloader properly first. If it still continues go hunting for air/leaks. I’ve had pressure trapped unloaders get hung up and continued pulsing until I reset them. Could have gotten a small piece of debris in it as well that is keeping it open/not bypassing fully causing pressure spikes. Have you tried running the machine without the gun to clear out any junk?
Yall ready for the answer…. Drum roll…. It was the freaking gun. It was a bad gun… changed out the gun and it was fine.
Glad you got it squared away. I’m curious, though… if it was the gun, why were you only experiencing the issue when pulling from the tote and not from the spigot? Sounds like there may have been more than one underlying issue, and getting everything sorted out finally cleared it all up.
The other issue was setting the unloader but once the gun issue was handled which is what was causing the cavitation then the gun was fine. Whenever I was running from the spigot it wasn’t relying on the suction which kinda bypassed the issues
