My experience is based upon two Senegals, both rescues. Both tended to be quieter birds and I think that may be true of most Senegals overall.
However.....
They have an occasional high pitched screech that can pierce your ear drums. It is loud and kind of painful. My first Senegal, Kelly, only did it when I was leaving. It was a sad "Don't go!" type of thing that broke my heart every time. Other than that, she hardly made a sound.
My current Senegal, Sidney, has a head splitting shriek that he does far too often. One of my Blue Crown Conures is a constant noise maker. She squawks continually and he responds to every squawk she makes with a shriek. Equivalent to a kid screaming "Shut up!" at his noise making siblings, I suppose. I put the Blue Crowns to bed earlier than Sid and he immediately quiets down to a few beeps, peeps, hoarse honks and a few other softer noises. If it were not for Tootsie's constant noise I think Sid would be a quiet bird. As I write this, Sid is preening on the arm of my couch and I don't think he has made a sound for at least 15 minutes.
FYI, the high pitch noise the Senegals make can go through walls a lot easier than the lower pitched Conures. I can hear Sidney down the street but can only hear Tootsie and Rosco when I get closer to the door.
As for your question of "sweetness", that is so much of an individual bird thing that is almost impossible to answer. Generally, Senegals have a bit of a reputation of being strong willed and can be powerful biters. They might have the 'pit bull' reputation of birds in some ways. Some of the best dogs I have ever known were pit bulls. They were raised right and showed none of the bad traits the breed is known for. But raised wrong and you have a dangerous animal on your hands.
My first Senegal came to me as a frightened wreck but she turned into the sweetest, most gentle bird I have ever known. I don't know too much of her history but a lot of careful treatment and she became a beauty. Her easy loving nature astonished people who knew her before I adopted her.
From what I know, Sidney was a bit of a neglected bird for the first 5 years of his life. Spent all day alone outside in a cage on a porch. He was not socialized well and as a result he is strong willed, independent, and when frustrated he can give a bite that is to the bone. He tests me every day in some manner to see what he can get away with. He will turn 10 in July and I keep hoping that he will calm down some with age. But no signs of that yet, unfortunately.
So in conclusion: Senegals are mostly quiet birds that seem to like a quiet environment. But like all parrots, they will have their noise making moments. Sweetness is individual, of course, but good hand raising will certainly increase the chances of having a loving bird.
Best of luck to you in finding "The One" in your search!