Tammi,
Looks like great minds think alike. About the second thing I did to this machine was replace the nylon thrust washer and set the lash. So I'm pretty sure it's not horizontal movement of the main shaft.
I've had both of them stripped to their bare undies so to speak sitting side by side and there is nothing that visually stands out. Other than one is a noisy thing. I even took the noisy one to my LSMG and he said there was too much clearance on the vertical shaft. But when I reduced it the motor could hardly turn the machine, and the noise was still there. So I loosened it just till there's enough to run free.
I've got a manual for the 538 and it's series of machines. Step by step diagnosis and adjustments for everything that moves. I need one for the 413 cos I sure can't sell it like this.
I'm about to tell my wife we're gonna run the noisy one till what ever is making the noise breaks. Run it till it blows up.
It probably won't though.
Joe