At 85k miles I've got what sounds to me like too much clearance in the valve train on one valve, You know, a tap-tap-tap at about half engine speed when under light or modest load. But, it clears up when boost gets to 20 PSI or so and isn't really noticeable at idle. The noise also goes away when I turn up the radio.

This is my first diesel and I've had it about 10K miles or so. The noise if anything got worse after hauling the 5K lb boat up the mountain to the lake. Could this be an injector? How would I tell which one(s) if so? There's no smoke on startup or oil level issues and I'm getting 17-18 MPG combined when empty.

I can go through and check the valve clearances, but I'd rather not invest the time if its "just a diesel thing." I'm hauling a 10K lb brick a thousand miles round trip to SoCal over the Grapevine next week and don't want any issues.

 

I have a similar clatter under load only. As soon as you let off the throttle it smooths out. Only does it when getting up to speed then smooth, Not a tapping sound however. More of a clatter. I have been told by dealer and reputable mechanic it's part of the 3rd injection event and it is normal. Mine did not do it when new and was told my engine loosened up a bit and is normal.

 

Mine does it too, sounds kinda like spark knock in a gasser. I don't let it bug me anymore ,truck runs good and pulls really hard so I'm saying it's normal.

 

Any update on the knock? What you described is exactly what my truck has been doing. Knock on light to med load then when under boost or heavy load it clears up. No rough idle and good power still. Sound gets louder as the truck warms up. Is also louder after pulling a trailer like you mentioned. Doesnt matter if the Smarty is on or not.

 

there is a Cummins bulletin that sounds like it relates to your problem, and it states that it might be from common rail injector ball seat erosion.

 

put in some seafoam, powerservice, or standyne, and go tow something heavy for the day. see if the noise goes away.