Need help from snapper experts

[afoulk]

Finally got my snapper self propelled mower that I bought a couple months ago running again and used it for the first time this weekend. It runs great now that I rebuilt the carb and adjusted the valves, but I'm getting a lot of vibration and noise from the self propelled system and the noise is driving me nuts. Mine is a newer style model with the solid rubber driven disk driving the differential belt, instead of the old aluminum ones with the rubber band around the outter edge. Anyways, I ran the engine once I got it fixed up an and held some emory cloth to the steel drive disk on the deck with it running to clean all the rust off of it, so I know that surface is ok. I noticed that whats going on is the rubber driven disk is jumping up and down, not enough to affect the way the mower moves, but enough to cause the linkage to rattle and drive me nuts. It clears up if I press down on the rubber disk guard, or almost clears up if I'm in 1st or 2nd, when the spring has the most tension on it. There is slight play in the steel drive disk, as well as the bearings on the drive shaft that the rubber disk slides on, but no more than my old snapper mower, and it doesn't have this issue. So, I'm thinking its either the spring loosing its tension, or maybe the rubber disk is worn down to far. Is there a measurement for that? Any other ideas? The metal disk gets pretty warm to the touch after mowing my 1/4 acre, I'm thinking from the rubber disk jumping around on it.