My 66 230SL had sat for 15+ years and the old style long fuel pump that was the original equipment was stuck. The fuel pump does not make any noise that I know of when the vanes are stuck to the bottom of the unit. Do not apply power for too long since it might burn the motor windings. When the pump is operating, it sounds like a fairly quiet water pump like you would have on an aquarium or fountain.
When I removed my pump, I sat the bottom of it where the pancake screws are, in a container full of PB Blaster. I let it sit for about a week. When I went to take off the screws, I tapped them with a screw driver and hammer lightly several times to hopefully break loose 48 years of gunk. Next, take a perfectly fitting screw driver in like new shape, and bear into the screw while turning. If your lucky, you will be able to remove all of the bottom pancake screws without any breaking off. If they do, you can still get them out using several different methods that you will find here on the forum.
Next I soaked the vane and bottom of the pump in alcohol for several days and followed it up with scrubbing with a toothbrush. The only item you may need is the bottom rubber seal. For some reason, I got lucky and the old one still seals. However, I did go to NAPA and found another seal that will most likely work if I ever need a new one.
Good Luck,
Tom L.
66 230SL Auto