Hi Ken,
I had the same problem with my '78 1-18 last May. The pulsator nut was IMPOSSIBLE to remove by hand. I even tried the "foot-in-the-tub" technique and it was somewhat awkward with the 1-18; I think with the shallower tub of the Rollermatic it will be more effective. I then got my impact wrench out and that worked wonders; I didn't even have to hold the tub! Definitely have one handy when working on these as you will use them quite often...I know I did!
It's probably best to get a hold of the appropriate Frigidaire Tech-Talk for this model line (what is the model lettering of your machine?), but the Repair-Master for the Rollermatics will also work as it focuses mainly on the solid-tub Rollermatic mechanism, even though the 1-18 is covered somewhat in there as well. However, try not to confuse the 1-18 diagrams with the solid-tub diagrams as they are two completely different mechanisms. Even then, Greg N. was telling me there were even different variations with the solid-tub Rollermatic mechanism down the model line!
After the agitator is out, you will see the water bellows below it. Check for any rips or holes. From what I've found, the bellows seals are made of heavy-duty rubber and hardly ever "wear down", so unless a nail gets in it, they will usually be in good shape and won't need replacement. If you want to remove the water bellows to examine the oil bellows, a set of Frigidaire bellows pliers is the best bet to remove the snap ring holding it on, but they're getting pretty hard to find. I heard one of those worm gear hose clamps will work, though. The rattling noise you hear is coming from somewhere in the transmission; probably the agitate arm/shaft/wheel assembly. Keep us posted on the progress; these machines are fairly easy to work on so I don't think you will have a problem.
Hope this helps!
--Austin