I have a Brent 674 grain cart purchased used this past spring. During wheat harvest everthing was fine. Shortly into the fall harvest we got a vibration when unloading, anything over about 1200 engine rpm shakes the entire tractor. We can't find anything wrong, bearings are good flighting is good auger is straight. The vibration isn't there with the auger folded back and is barely noticeable when the auger folded out and empty. It's with the auger out and loaded that the vibration becomes so bad. Any ideas?