Low frequency rumble

NE1 else have a low frequency rumble at light throttle about 50-65mph. Occurs infrequently and unrelated to the road surface, it is felt through the seat more than heard, though it is audible. Sometimes hard to tell if present at all but the confirmatory sign is the rear view mirror vibrates when it occurs blurring the image so I know exactly when it happens. This may sound silly but it feels as if it's running on 4 cylinders.