Sheep are herd animals and keeping one by itself will be stressful to it. So yes it will likely be loud or try to escape. A single sheep is not a good choice for being incognito. Even when they are not alone, ewes with lambs spend a lot of time calling for their lambs to follow them. Sheep will produce milk, meat and fibre, but no breed of sheep does all 3 well. The milk sheep will be skinny, have less robust lambs, and not so nice wool, some may have reasonably meaty lambs. The sheep with nice wool are mostly dual purpose as meat breeds, these will have just enough milk for their lambs. There are also strictly meat breeds, that have lots of lambs, not so nice wool, and just enough milk to
feed all their lambs.
I think you need to decide on priorites to decide what livestock you want. If you want milk and meat, there are some small milking goat breeds, such as the nigerian dwarf, or kinder. Goats would be as loud as sheep. If you want fibre and meat, and a completely undetectable animal, angora
rabbits would be worth looking at. Rabbits eat green stuff too.