"I don't mean to dismiss you, but it definitely sounds more like arcing, as kirbyvertibles suggested."
Brushes arching on the commutator makes a "hissing" or sputtering sound if the brushes and/or commutator are really worn.
Another thought . . . If the brushes are really worn down to where the spring tension is greatly decreased, the brushes will bounce especially if the commutator is very worn or out of round. This would explain the noise at start-up, that diminishes afterward as the brush float dampens out.
