To loosen the belt find a big screw on an adjustable dodad attached to the motor. Loosen the screw and the motor will wiggle around. Mine sew best if the belt isn't tight - just snuggish. I had the reverse problem with a very similar machine - same color, etc, different badge. It started sewing at a nice speed then slowed down. It took a hammer to remove the cover to the foot pedal - no screws, just a slide off - HA! Not when it's been on 60 years. I looked it over, it appeared okay, just dusty, so I vacuumed it with the dusting brush of my vacuum. End of problem.
Edit after reading more posts: The hum may be a motor problem or the belt may be too tight. Try loosening it. Are you turning the handwheel toward you a bit to start the machine? Some oldies like this. My foot pedal appeared to riveted - it wasn't. The cover slides (and I use that term loosely) in grooves on either outer edge of the bottom. Enter the hammer.
I worked on a Pfaff at my church that screamed when I tried to sew after a good cleaning. The belt was old and stretched out and was too LOOSE. Dropped the motor to increase the tension and the motor quieted down. Now to find a new belt large enough.
Last edited by irishrose; 06-15-2012 at 07:34 PM.