sorry I haven't replied to anyone, been out of town for a few days. I think there are 1800 or so engine hours on it. I don't think it's a valvetrain or injector, because it runs very smoothly under load, with plenty of power. Not much smoke except when you first kick the rotor in gear and rev it up, and that's because it's been turned up. It doesn't blow smoke or anything when it's making the noise, but sounds like it's missing. We took the pump off last fall during a break in harvest and got it checked out, they didn't find anything, but it still does it. I think we're going to take it to another place because the mechanic we've had look at it still thinks it's in the pump.