Edit: Just watched my vid, got that sound captured perfect!

Trying to pin this brand new sound down, Friday morning I had a perfectly 'normal' cobalt and Friday afternoon when the misses picked me up I immediately noticed a rattle from under hood.

Pulled off the valve cover, as it sounded like it was coming from the timing chain area but nothing looked amiss. New chain and tensioner when I built my zzp block this past summer.

All the lifters and rollers looked solid, cam caps were tight. If it is head related, nothing jumps out as obvious.

Then i thought maybe acc. belt tensioner, with the zzp SC kit they tend to crack and maybe the bearing was pooched or the spring was busted. It is solid, as much tension on the belt as there ever was and it floats nicely as it idles.

I'm down to exhaust leaks at the flange or a pooched water pump. I know my exhaust could use some TLC and the gasket between the cat back and dp is leaking a bit, that sound I know well enough as an occasional tap-tap-tap kind of rattle.

The water pump is plausible, one thing I've noticed is temps tend to drop FAST upon acceleration but temps never rise beyond 90*C (190ish F) but like I said can drop 20*F in a matter of seconds so that makes me a bit suspicious that the impeller or bearing is wearing out.

Have a short 30 second vid, anyone heard this before? I have to say it did seem loudest and clearest when I was underneath. Using a long screwdriver as a stethoscope for the first time was interesting...but couldn't really tell where it was coming from.

 

interesting

 

do you have the zzp turbo manifold?
theres a cobalt i worked on that had a big turbo and made that sound when the manifold warped causing a huge exhaust leak, and made a rattling sound like that when it was cold. that was before they revised it and put relief cuts in it. the new one i put on hasent warped...yet

wish i took a video of it

 

Whoa! No I'm supercharged, but the exhaust manifold is a ZZP long header unit designed for the LSJ. I'll be pulling it this week or next (whenever it ******* stops snowing fer christ sakes) to have a look. I've already replaced the DP section of the long header for $250 after barely 18 months so it wouldn't surprise me if it was that.

 

Mine sounded like this. Idle was quiet, the sound you hear is the injectors. At 2500 and up it was ratcheting. The main guide bolt was sheared off. Replaced it with a 10.9 grade bolt.

 

God I hate when this happens.. Mine broke as well and got stuck between the chaine side wall of the head. Which jumped timing, and blew my motor... So make sure you guys really check, and dont just assume.. I would have done a million things differently had I had the knowledge, and skill set to go along.. For the OP well written, and very nice advise hopefully alot of people who do this can follow your foot steps. GL men.