Hey, I believe I made a thread about this a long time ago but never solved the case and didnt link any audio. The engine has 116k miles and started doing this after over-revving due to a misshift at high RPM (3rd to 2nd...). This was over a year ago and at first it only ever did it when cold so I figured maybe I just encouraged the well known piston slap to start early, but for a while now it's been doing it intermittently at idle even when fully warm. Never does it above around 1000 RPM. Pulled the valve covers and pushrods, replaced the pushrods just in case, but that didnt change anything. I've stupidly been daily driving it this whole time and performance doesnt seem to be affected though I'm not good at judging these things. Did a cold compression test and all cylinders are about the same with minor differences. Lifter, piston? Thanks in advance:

 

You try pushing the clutch in, while it’s doing this? Sounds more like a vibration in the vid than anything, but it’s impossible to diagnose these things over the internet.

 

It still does it with the clutch pushed in, yes. I would be surprised if it wasn't an internal issue though, the sound increases in speed with RPM until it goes away after 1000ish rpm. I guess it is possible the cause was always present but maybe I destroyed the motor mounts when I over revved it. The sound is much more noticeable from the interior and I do feel a vibration when it occurs.

 

Check all pulleys. Remove the belts and check the pulleys. You could run the engine for a few seconds without the belts to see if the noise changes or isn't there. Possibly bad harmonic balancer

 

Definitely sounds like vibrations causing the noise. Can you get us an audio clip from outside the car and under the hood?

 

I'll try to get a clip next time it's making the noise, to me sounds loudest around the driver side head but who knows.
Funny you mentioned that, my harmonic balancer has had a wobble to it since I bought the car. I have an ATI dampener I ordered in the mail I've been lazy about installed, currently waiting on an install tool in the mail. I doesnt really sound like it's coming from the balancer but who knows maybe the sound is echoing around. I'll report back after changing it out, thanks

 

Check the catalytic convertors, one of mine sounds like a 14" spray can rattling! Hit it with a rubber hammer to see if the honey cone liner is loose.

 

You say it sounds loudest around the drivers side head. Does it sound internal or external? I can't think of much internally that would make a noise like that and be intermittent. My lifters tapped a lot before I tore my motor down, but that was because they had 170k on them.

 

Sounds more like a vibration rattle in the exhaust.

 

Headers? Might be rubbing the K member.

 

That or the exhaust pipe off the collector on the drivers side where it has to make that turn as there's not much room there.

 

On mine the noise 1st started only when cold like yours. I thought it was valve train noise, but it was the noise bouncing up the headers. So have you beat on the cats yet?