So after years of driving only LS based trucks, doing some work to my original 5.3 tahoe and now my 6.0 denali, I broke down, sold my daily driver, and bought a 98 Z28. My 96 cobra is jealous to say the least. 136k miles, brand new 3200 Stall and RPM Level V A4.

But, after I got it home, I did some driving, and noticed an occasional knock. The more it warmed up the more it happened. But it only appears under light acceleration and from 1500 RPM to 2700 RPM. When I accelerate hard it is pretty much non exsistant and is not apparent at idle.

Things previous reading and LS experience has told me, this is not piston slap, my tahoe was the king of piston slap when cold I cant miss that sound. Also I know that it sounds more "light" of a knock than when my 4.2 Trailblazer threw a bearing.

My next step is to change the oil and cut the filter open and inspect, and also pull the wires one cylinder at a time attempting to narrow down where it is coming from. Any suggestions would be insanely appreciated, I have big plans for this car and will be selling the Cobra soon to fund the build but really dont want to have to do a whole new short block this soon.

VIDEOS!

 

That doesn't sound like rod knock to me, did you check rockers/lifters?

checking the filter is a good idea, but I'd pull valve covers and go from there

 

If this is a auto it MIGHT be the converter bolts are loose,maybe a cracked flywheel. I alway start with the easy stuff first

 

I have not pulled the valve covers yet but thats in the plan for tonight, anything I should specifically look for? Maybe play in the rockers or springs being broken or weak?

This is something I also thought of, the trans and convertor only went in 2,000 miles ago and the previous owner naturally didn't mention any of this noise to me. Is there a cover i could pull to look for any signs of this? This is my first built auto, I usually dont touch them but finding one with a built trans made me reconsider.

 

^^^ This was my first thought.

 

☝ I'd check that first, maybe previous owner didn't install it properly. Check that first

 

Alright so I did some initial testing over my lunch break. I disconnected each injector one by one and revved to 1900 RPM where the knock is loudest. Turning off the injectors did not really point to any problem cylinders. Cylinder 4 seemed to make the absolute slightest difference in sound, but it could just be me. If anything it changed the tone ever so slightly.

I am posting the video of this, first rev is all cylinders, second rev cyl 4 is unplugged, third rev all cyl.

Also, I am finding it weird that it still does not make an audible knock at start up or idle, and that the knock is most pronounced at 1900 RPM.

Oil Pressue is just under 40PSI sitting at idle, drops to 25PSI in gear, and goes above 40 when accelerating based off the stock gauge.

 

Tough call from here but it may be lifter noise, try adding 3 ozs of TRANSX. You can buy it at walmart. If its lifters you should notice a big difference, if its a bearing it won't help. Also I would pull the oil filter and cut it open then lay the pleats out to see what may be in there. BUTT looking in the filter may not show anything even if a bearing is damaged because it may just be flattened or pounded thin. Aluminum engine noises can be tough to diagnose.

 

sounds valve train related.

 

SOLVED

I appreciate all the help! Finally got this solved. It was cylinder #4 rod knock. Went and bought a low mileage 2006 LQ4, ordered a tsp torquer v4 kit for it, will get it swapped sometime before summer.

 

I had the same problem ended up being the f***** air pump hose on the driver side near the firewall, the hose had a small rip in it, used some gorilla tape and the noise went away. It tricked me and my fellow gear heads too, the knock was loud! To anyone curious pull off the hose and it will make that same noise.