Not very noisy at all. I can sleep in a tent with them next to me and I'm not a deep sleeper. You won't hear it in the boot whilst driving etc that's for sure. They are barely audible with the engine off.
They will cool stuff but don't expect it to be quick like a fridge if something is warm. I plug mine into the mains the night before and leave it running overnight whilst empty to cool properly. I then put the food & drinks in the next morning and stick 2 of those small freezer blocks inside as well. Everything then stays fridge cold all day.
To give an idea of how well the Mobicool ones work I've used both to store milk in whilst camping for at least a week (basically until it ran out in normal use) and it was fine all week on both occasions. When not running they will still keep stuff cool for a good 6-8hrs