I recently bought an automatic 1991 firebird formula TPI 5.7, with around 106000 miles. around a week ago i started to hear a light knocking sound while it was in idle, but only every once in a while. now it is constant, and it does move (faster and slower) with the engine. the sound is pretty faint, and cant be heard looking at the engine. the best place to hear it is in the drivers seat, or looking under the car right behind the front wheels. the sound is pretty rhythmic, and stays in neutral. the only place you can really notice it is in the cabin, but it does get a little louder as the engine heats up. Any ideas as to what is making the knocking, or if its serious?
Thanks.

 

its your rod hitting the block. could be VERY serious. if neglected it will lead to engine failure. check your oil. sometimes having low oil can cause a knock. id also say go ahead and change your oil to, put in like 10w-40 or even a thicker weight. see if that fixes it. sometimes it will sometimes it wont. if it doesnt its going to need an engine rebuild or a new engine.

 

I certainly hope, for your sake, that it's not the sound of a rod hitting the block. That would cause an instant, and catastrophic failure. More likely it is a rod knocking against a worn crankshaft journal. This can be caused by excessively worn rod bearings, or a rod whose big-end has been damaged and is out of round. Before you tear into the engine however i would eliminate all other possibilities. You didn't really describe the nature of the sound. If its a light ticking, say the sound of a dinner spoon hitting a granite counter top, then it could be as simple as excessive valve lash. If it sounds more like the sound made when you hit an anvil with a hammer, then i would guess rod knock. If it sounds more like a "pop" of air it could be an exhaust leak.

Care to describe your sound in more detail?

-Zander

 

He says it doesn't sync with the RPM's (get faster or slower with the rev of the motor). Wouldn't that rule out any rod knock or valvetrain issue? Sounds strange.

 

Timing chain slap?

 

Its a light tapping. the sound cant be heard standing five feet from the car, but when you're inside it becomes very apparent. and it does go faster and slower with the engine, and the sound is still present in neutral. is it possible that its the stall converter? i looked underneath, and the flywheel cover is missing...

 

Well heres one way to find out for sure if its in the bottom end. Run the car and pull each sparkplug wire one at a time (with a set of good insulated pliers so you dont get zapped) and if the sound changes when you pull one of the plug wires you have a rod knock on the cylinder. Sounds like its probably a rod knock but it could be a multitude of other things like a set of headers thats not tight to the block. So dont panic yet start by pulling the plug wires and youll know right away if you have bottom end problems. If its turns out to be a rod knock you have serious trouble and if you have any desire to save the motor stop drving it immediatly. But thats the worst possable scnario if its just about anything else but that ide say your in pretty good shape as far as repairing whatever it is without having to remove the motor or having big time machine work done.

 

In addition to Rolling Thunder, if the noise does not change, and you find the bolts are loose on the headers (or exhaust manifolds) to the block, you will need to replace the exhaust gaskets and make sure everything is as tight as possible.

 

my bad ya'll i thought it said the knocking stayed with the RPM's. me readin to fast on accident.

 

I've narrowed the sound down to the tranny. I know for sure it's not the motor.. The sounds gotten a little louder since, but barely noticeable. When in the drivers seat, the sound comes from under the middle of the dash. It's a light tapping sound that stays with the Rpms and doesn't get louder with load. The guy I bought it from must have known, cuz he switched out the stall converter. But it didn't help.. Any ideas on what the sound is? Thanks for your input guys...

 

check for a cracked flywheel