So while mowing tonight the 855 suddenly got a nasty vibration. When I bought it it seemed pretty smooth, especially for an old tractor, a month or so ago it started with a little vibration, nothing worrisome, and really only felt in the foot rests. Tonight while I was mowing it got worse all of a sudden. I still only feel it in the foot rests, but now it is like standing on one of those shiatsu massage machines. At first, when it started I stopped and checked the oil, which was a bit low, so I topped it up, and the vibration went away for a while, but came back.
Any ideas where I should start looking? I'm going to bring my timing light home tomorrow and check that.