I took my 51 3100 on about a 30 mile round trip to a cruise in. This is the farthest I have driven it since I've owned it. (Only a few months) I noticed that at partial throttle I have what can best be described as a rattle. From the cab it sounds like it is toward the back of the engine. When I let off the throttle it goes away and when I accelerate it goes away. I have good oil pressure. I experimented with pushing in the clutch to see if it was related to the clutch but with the clutch pushed in it still makes the sound.

Sorry for the long post. Any ideas are appreciated.

Sincerely,
Chris