They make some noise, but it's hard to get an objective measurement of noise level. Some people complain it's too noisy if they can put their head next to it and barely hear it, others as long as it's not obtrusive in another room it's not too noisy.
You also can't compare noise levels of internal and external filters, internals have a bunch of water damping the sound, externals don't. They're almost always totally inaudible unless they're touching the glass.
No, the yellow testing liquid is blue. The first drop should turn the water blue if the KH is higher than 1° When the concentration of the reagent is lower than a given point it is blue. This point increases as the hardness of the water increases. Once the concentration increases past this point it turns yellow.
If the first drop was yellow, after flipping it once or twice to mix, then it sounds like your kh is less than 1. How did you get 3-6? You measure from when it changes from blue to yellow, not from yellow to really yellow.
I was told you're not required to report single car accidents without injury. And the only reason to tell the cops is if you think your insurance excess is too low and you'd like to pay a fine on top of it.
You don't need to add exactly 6 guppies worth, if you do you're doing it wrong. I'd guess they'd be in the region of 1 drop per week, you should be adding more like a handful of drops per day.
Two best options:
1. The old fashioned method, Add a couple fish, feed fish, add more fish in a few weeks.
2. Fishless cycle, Add drops of ammonia for a couple weeks, then add a bunch of fish.
Don't bother with stresszyme, cycle, etc.