Z06supercharged

Before I pull the valve covers to check things this is whats going on with this engine.

Start engine cold at 45-75 range degrees ambient temp and it makes no noise at all for first 30 seconds then the light tapping/knocking appears off and on at idle most of the time. Its been making this noise for 1.5 years and 10k+ miles.
Have changed oil 3 times using 5-30 and absolutely nothing in the drain pan or on the magnetic oil plug to show something going like a bearing and cut open the oil filters and nothing at all in the pleats bearing material wise or anything else incriminating evidence wise of a possible failure inside.

Sometimes it makes no noise at all at idle or off idle and up but usually its there at idle and up to about 1600 rpms then its gone. If I do a few wot romps and go down to an idle the noise is usually gone. If I barely tap the throttle at idle 50 rpms up you hear the slight knock instantly for a split second and its gone, gradually bring up or rev fast rpms in neutral and really no loudness.
In 6th gear at 1600 rpms you can barely hear it tapping like maybe an exhaust leak or lifter but its not anywhere near each engine revolution and maybe close to every other revolution and it goes away with higher rpms but if you slowly give it throttle say at 1200 rpms on freeway it gets louder until about 1600 and goes away. We have been under, over and beside the engine with stethoscopes listening and cant detect where its coming from and even though you can hear it at idle inside the car and beside the car echoing we cannot find where it is coming from and its driving us nuts.

If a/c compressor starts the knocking becomes much louder, not a different rhythm or faster noise just louder just much louder. Doesnt do it all the time though but mostly it does with a/c on and if it does it stops after a minute or so being louder and then noise goes back to less loud. I heard the fans come on and the knocking became a little louder as well so was thinking alternator brush bad perhaps when load is put on it. Unplugged alt and noise still there. Checked a/c compressor and its tensioner/pulley/clutch and all appears ok as well. Balancer is good no wobbles etc. In a few weeks ill remove the a/c belt to rule that out completely.

When shutting down engine (clutch pedal in so no noise from that), its has a very loud pronounced noise winding down heard at 58 seconds in video before its done stopped.

Piston slap maybe but that usually goes away when warmed up.
Oil pressure at idle is 30-35 and never fluctuates at idle when the knock is present.
Thinking one of the valve springs broke (has double springs).
Still possible a tensioner somewhere but didnt detect noise from any of them when noise is present.

Rod knock would be much much louder and would be exactly every other revolution and gets louder as miles go on and be all the time but when under a heavy load dont hear any noise at all increasing with engine rpm and spins to 6800 with no noises and smooth.
Maybe wrist pin or piston skirt hitting?
Runs excellent though, no engine misfires in logs pointing to anything, logs dont show any abnormal knocking being picked up at idle when its heard. Vacuum is good at idle and steady at 18.

Why turning on A/C or fans coming on making noise louder cept for putting a load on engine or electrical? You can really hear it when letting clutch out taking off when load put on engine and if you let rpms drop a little with clutch release you can really hear it well but it doesnt match engine revolutions and doesn't even sound like every other either.

You absolutely cant hear it under open hood when running but can hear it echoing under car and to the sides and when shutdown you can hear it loud and clear as it rotates down no spark as if its right on top of front of intake area but cant tell exactly.

Looks like its gonna be one of those it finally showed its ugly face and its time for a new engine.

Here is a sound clip although not very good but it was all I had for webcam on laptop. You can kinda hear when I hit the a/c button off and on as it gets somewhat louder more pronounced then quieter when a/c off. Tried to get sound of shutting down noise as well but hard to hear it but its at 57 seconds..
http://s144.photobucket.com/albums/r...ent=Video2.mp4

bumble-z

Mehanic's stephoscope, should be able to pin point area of concern.
Good luck.

Z06supercharged

Myself and 3 other people have used em and still cant pinpoint it. Can hear valvetrain touching valve covers and sounds good, injector areas and side of heads and front and back with it and all sounds good, touched intake all over and each runner and all sounds good, touched every exhaust port area, spark plug area, every area of oil pan, side of block torque tube thinking bad bearing there and all sounds good but you can still hear the noise inside the car and when shutting off car. We are still listening with scopes but have yet to pinpoint it at shutdown but quest continues.
A.I.R. pump is off the car but valves remain so I touched each valve and no noises there so they remain closed and large air pump hose plugged off. Someone mentioned evap purge valves maybe but those are in back at tank so dont think so.
I travel a lot in retirement and dont spend much time on the car anymore trying to pinpoint crap like this. Hell I almost wish whatever it was would just break and id yank the engine and go bigger. I'm flying back out tomorrow so the car will sit another couple weeks but then im changing injectors when I get back and will pull valve covers and such to see whats up in there if anything and do a cylinder balance test with HPTuners.

Eric D

Managed a double post some how...

Eric D

I played your video, but I really can't pick out what you are hearing. However, I want to point out that if you hear it more from inside the passenger compartment you could be experiencing what is called a ground out. A ground out can be one of the a/c lines not isolated properly from the body. The fuel line can do the same thing. It will also vary at times with intensity.

Z06supercharged

Thanks and sorry for the quick lousy video sound. Ill check those lines.
Just re-checked oil pan and those lines with mechanics stethoscope and all good, shut car off and we all heard it loud and clear and half agreed it now sounds like pinpointed to front of engine upper part and some of us said no its back further.... Its like chasing a greased pig as it obviously echoes through the whole engine area...

dgrant3830

Any number of possibilities in there. First thought would be a lifter having issues but the only way to find out would be to replace them all. With your few miles on it, its unlikely that a lifter would fail this early. The oil pump is there under the front cover with the timing chain. If you aren't hearing it at a valve cover, its probably not a rocker arm as they can lose roller bearings from time to time but its worth a look anyway. Now I'm not an expert on dual springs but my simple thoughts are if one spring had broken and another still working, the engine might not react badly right away whereas a single spring failure setup would cause instant issues. Possibly a sticking valve guide or slightly bent pushrod...

Z06supercharged

I'm out of town now for a while but have thoughts on it being the evap solenoid gone bad and will check it when I get back.

Eric D

I look forward to your update.

Z06supercharged

Ended up being a lifter thats bleeding down at low rpms. Finally just did my idle down to 600 and it was clear what it was.

All_Motor_C5LS6

I had the same sound. Turned out to be a sticky lifter. I added marvel mystery oil to the crankcase to free the sticky lifter and that sound went away