knocking or slapping noise 05 volvo ved12. please help!!!!!

I last drove my truck a week ago today and it was fine. No noise and ran great. checked my oil today and it was full. started my truck up today and its making a knocking noise. if I rev it a bit and get to 1200 rpms it goes away but comes back after its lower than 1200 rpms. kind of running crappy too. its not blowing any smoke. Anyone have this same problem with their truck. any help would be greatly appreciated. 05 Volvo with a ved 12 and it has 739000 miles on it. the mechanic at the garage I park at said he thinks it might be a bad injector and that it has a dead cylinder. Could that be right? Im panicking here!

 

I also forgot to mention that there is no codes or check engine light on

 

Talk to http://www.thetruckersreport.com/truckingindustryforum/members/50847-pablo-ua/ genius with these engines

 

Thank you very much. I sure will

 

If it's not blowing smoke is a good sign. If the mains were shot the cylinders take on an egg shape, wear the rings out and you'll get blow by and burn oil. It's probably a dead cylinder like he said.

 

Thanks condo cruiser! You made me feel a lot better!

 

Does it have Jakes on it? Sometimes the leaf spring breaks and causes the engine brake rocker arm slap between valve bridge and the cam lobe. Super common if it's been a while since an overhead set. Also could be a cam issue. No smoke tells me it's probably not an egr or fuel issue. Usually if it is knocking due to injector it would be smoking white. Same with a stuck open egr causing overfueling, you can smell fuel.

 

Yeah it does have a jake on it.

 

If the noise sounds like it's under the valve cover check the leaf springs. Keeps the rocker from slapping.

 

Is that expensive fix duramaxxed?