Couple questions.
What salt are you using?
You said you don't have any test kits, but do you have a refractometer, or at least a hydrometer to measure salinity? If so, what is it?
Did you fill the tank with tap water?
In the few days you had it running, how much water evaporated and did you add any top off water? If so, was it tap water?
My guess is, and it depends on all the above:
High ca salt mix. Some are very high...maybe too high in some situations. If you don't know your salinity, that may be high. If you are using tap water, it may be high in ca also. You don't know your pH. If any water evaporated and you didn't replace it with fresh water, all the parameters in your tank got higher.
So, it is very possible you use salt that's high in ca, your salinity may be high, tap water was used, water evaporated but not replaced and your pH is high.
Add this all up and you have a calcium precipitation event. Basically the water couldn't hold the amount of calcium and carbonates in solution, and it precipitated. It will all settle in a day or so, but your ca, alk and mag levels will be way down.
Oh, and for your overflow, the valves should be opened all the way all the time. If you have those strainers pointing up, it will always be loud. Take a look at
this durso. It's in an overflow box, but that can just as well be drilled like your tank. Note the position of the elbo inside the overflow box.