Mine makes the same "chucka, chucka" noise as Mitch's mom's machine. It drove me crazy as I thought it was in need of more oil. But it's just making a noise.
The bobbin on the 15-91 is an oscillating hook so it moves back and forth. If the circular parts that hold the hook in are loose they will rock back and forth and make a racket. Check them and make sure they fit tightly.
Joe