Exactly what happened on my 9660sts last season. Mechanic did not put the left front cover back properly and all the chaff blew out thought that gap. Only after close inspection, did i see the problem. Another indicator was the wheat sample that I couldn't get clean enough. Direct result of the wind escaping at the rotor cover door.Make sure your plastic side rotor cover doors are closed properly and tucked under the channel they are supposed to be under. This is the only time we get any chaff under the hopper. The easiest way to clean this mess is with a gas powered leaf blower.