One thing I have noticed about second-hand "S" cams, whether from Hitchcock's or the set I picked up on eBay a few weeks ago, is that the bronze bushes can be worn. While they can look OK, if they're sloppy on the spindles that won't help to keep the clatter down, and might be enough to cause the rattle you didn't have before when using cams with un-worn bushes. Attempting to adjust the cam gear lash under those circumstances won't quieten it down. As you say you have done everything else as per instructions, that's where I'd look next.
Not a huge problem if that is the case, you need a new set of cam bushes, some way of pressing the old ones out and the new ones in, and a 5/8" reamer.
Good hunting.
A.