Strange magneto noise-C progress

I got the C together today using parts from the other C, which is now almost all dismantled. I swapped the entire steering assembly and the front end, the radiator and thermostat housing, the sheet metal and the starter. I still need to put the bolts in the front of the hood and get a shorter fan belt and some threaded rod and put on the alternator and wiring. When I started it up and drove it out of the shed everything seemed fine except that there was a humming or buzzing noise coming from the magneto which was audible with the engine running. The mag is a Fairbanks Morse K4B4A rather than the J4B3 which I am used to seeing. It is a bit bigger and is the cleanest, shiniest magneto I have ever personally seen, it looks like the pictures of reman ones I see online. When I put my hand on the mag case it seemed to be vibrating but I could not tell if it was vibrating more than the rest of the engine. I would have thought the sound was a water pump bearing going bad but with no fan belt that didn't seem possible. The sound did seem to be coming from the mag area but I could have been wrong. I only ran it for a couple of minutes and then shut it down. Any advice will be much appreciated.
Zach

K4B was standard on early AC's and very different inside than the J4. Pull the cap off and the problem may be self evident. The giant disc is the rotor with carbon brush riding on the top like a record. the rotor must be tight and turning flat. The brush riding on it needs to be in good condition and not too short. The outline on the rotor from the brush must be flat and smooth. They are a good mag but I don't like them because parts are not as easily found. good luck!

Grandpa's WC had a FM mag on it for years. Nice looking little C you have. What's the story on it? No 300 parts yet?

Glenn

I will take a educated guess on this one. May have bearings that are bad or have sat a long time and rusty??? Would like to see a picture of it just in case it is the other common mag that you may not have to manual for unless you got both or them from me back in if I remember right 2009??

Thank you, I will open it up later today and check it out. The disk is a new one on me.
Zach

I put this C together with parts from two Cs. The 300 is waiting on parts to come in the mail and then I can start to put it back together. I didn't want to waste the heat wave that we had here, yesterday the temperature got up in the 50s and melted most of the snow.
Zach

Thanks, I will have to check on the other computer where the manual is and see if it is covered.
Zach