I have a 2006 722D that is making a noise I just can't figure out so I'm looking for some help. The noise is coming from the rear of the mower and can be best described as a metal knock or a metal vibration. This has been intermittent and sometimes doesn't appear at all. It only happens with pto engaged and usually after mowing for a while. Possibly as something heats up? As soon as you shut pto off it goes away. I've checked everything I can think of with no luck. The mower has had a new clutch installed this spring and made the sound both before and after. I don't believe its a engine knock because I would think it would happen with deck on or off. The mower runs temp wise in the green and doesn't burn coolant. Oil looks good with no metal in it. I have checked motor mounts which look good. The mower does smoke a little more when this happens but I see no difference in power. Could a bad injector do this under load? Any input would be much appreciated.