i have a 98 1500 5.9 115k. the truck runs great after it has warmed up and i dont get on it, however if i get on it a little and push over 2700 to 3000 rpms i get a horrible grinding clanking clattering crackling sound comming from behind the dash. i have not been able to locate the problem yet. it is even worse when it is cold and feels like i have almost no power.
anyone else had this problem any ideas?

p.s. i have not replaced the plenum, i looked inthere with a flashlight and couldnt see any oil

 

When you pop the hood and the engine is running at around that 2700-3000 RPM mark, is the noise present in the engine compartment or are you certain it's in the dash?

I got a clacking sound coming from my truck that resonates from my passenger side valve cover. A mechanic said that it could be a hydraulic lifter seal that went bad. Since it's losing pressure, it's not holding up against the cam thus causing the clackage from excess play.

Be sure you know where that sound is coming from before anything. Get a long screwdriver and use it like a stethascope. Should tell you if the sound's coming from the engine for sure and if not, you'll know where not to focus your attention.

Last but not least, noise for me often goes away temporarily when I use some Lucas Oil Stabilizer in the Crankcase. Mabye try that next time you get an oil change for some temporary relief if indeed it is an engine related noise.

 

it is coming from inside the engine itself, tried lucas and premium gas it may help but cant be for sure it is never constant sometimes it is loud others quite sometimes it lasts for a while others it will stop in seconds. o me it almost seems like there is a baLL BEARING floating around in the engine.

 

Do the automotive stethoscope thing and locate exactly where the sound is coming from. I have a feeling those sounds might be from a wiped out catalytic converter. Once it breaks down the honeycomb pieces rattle pretty good and the loss of power is very noticeable. You should be able to hear the sound from the cat even under the 2700 to 3000 rpm range.