First guess would be a loose rocker arm......if it goes away at time it is most likely an exhaust leak and or a cracked exhaust gasket.

 

Crankshaft maybe ?

 

I've noticed that if its doing it and I really stomp on the gas hard, it goes away and doesn't come back for awhile. The first time it happened I opened it up on a back road and it went away for awhile.

 

If it goes away, it's not the crank..crank noise stays..u might have a sticky valve somewhere..

 

I'm hoping for something fairly minor like a bad exhaust gasket somewhere.

 

After hearing that sound I'm pretty sure yours is a knock. Hope that dealer can locate it for you. I'd hate for you to miss out on that sweet Adventure pkg Alpha.

I have a 06 I-5 with more of a loud rattle sound at about 20 mph and worse around 40 mph right side--steady with right accelerator position. Took off my loose skid plate (lube place forgot 2 of 4 bolts) but noise still there so I'm thinking it's my exhaust manifold now but not having luck pinpointing it.

 

Have you double checked your heat shield around the exhaust manifold for rattle?

 

Knock was back again last night, still not sure what it is. Have an appointment to take it in for service next Monday to a new dealer, super sick of my old one, tired of them playing guessing games and getting the truck back with nothing done.

 

Your 2008 H3 Alpha is setup to idle at 550 RPM when warm. The frequency of the sound at the beginning at your sound file is 4.56 Hz. Here is some frequencies at idle:

Frequency at idle = 9.17 Hz

Frequency of a single cylinder firing = 4.58 Hz

Frequency of all cylinders = 36.67 Hz

The sound you are hearing is not related to the crank frequency but related to the cam frequency or a single cylinder making noise. If there is power loss or a rough idle, then maybe it is a head gasket blown for one cylinder. If a there is no power loss, then perhpas the exhaust manifold gasket is leaking for one cylinder. The rear cylinders for the LS engines usually run hotter (these are the first to knock) - I'd check these first.

The LS engines had some cylinder knock issues during warmup, but this usually went away when the engine heated up.

To narrow things down, you can kill individual cylinders as see which cylinder is producing the noise.