Well, if the note does not change when the others do, it might be in the percussion track. What you might do is selectively solo (or mute) tracks until you isolate the track that it is in. If it is in the drum track, then you have to find the percussion instrument that is causing the problem. If you can, listen to the original style and try to figure out what that instrument was supposed to be. Then reassign either in the midi file or program a new kit.