C ???help. sounds much like a muffled jake brake on one cylinder. more noticeable under coast i.e. going down hill-kind of fromp fromp sound. I have been chasing this noise for some 2 months. Much more pronounced when all warmed up.This noise comes with a vibration sort of like one cynder missing.Replaced the injectors replaced clutch and the vibration dampner.All 6 cylinders are working-runs&pulls good.this noisecan be herd if quickly reved and let the throttle off also.Can be down with clutch released or not.Thanks in advance for your thoughts.If you think that you may positivly know the ans. feel free to as I am lost and the next thing is exploritory surgery! 1 800865 5200. Truck is a 1998.5 24 vavle 1 ton dually.Could this be the first sighn of piston going bad?

 

It may help us to diagnose the problem if we knew the year, model etc of your truck.

 

Can't quite figure it out from your post, but are you saying that you can hear this noise with the engine on but not moving? My first thought after I read your post was that you had an out of round tire. Have you tried running the truck when it is making the noise, slipping it into neutral and turning the engine off? Make sure that you are on a straight road when you do this to avoid locking up the steering. As soon as the engine stops turn the key on again to unlock the steering. If you can hear the noise with the engine off then it is running gear or tires. I doubt if it is a piston starting to go bad as you thought. That would be more of a ticking or slapping noise. Does the noise vary according to engine speed or to road speed? Let us know and we will go from there.

 

most predomint@ 1700 rpm , is most diffinly rpm related.