Rattle on acceleration 2005 Cadillac CTS

The car is a 2005 3.6l auto transmission. 282 000 kms on the odo and the rattle started at around 177 000 kms. I'm a salesman on the road so that's why there's so many mileage on the car.

To make the noise, the car has to been accelerating (not on park or neutral), the engine has to be in normal temp. The rattle only exists when there's light to medium throttle at 1750 rpms to 2400 rpms max and disappear if higher or lower than this. During the winter where the temp drops below 15 C, the noise goes away.

I've changed fuel filter, transmission filter, transmission fluid (every 50k) and always looks pretty good red when changing it.
Engine oil always changed, good results with UOA, always used synthetic, never drop below min on the dipstick.
I've changed timing chains and tensioners to make sure but the sound is still there. I've changed sparkplugs, MAF sensor, air filter, compartment air filter, transmission hoses, changed the rad.

Other than that, the car runs like a charm.

Here's a video of my car rattling:

the rattle starts at 0:13 on the video. I'm getting tired of hearing this noise. I've been fueling with 91 and 94 octane and still (as you can hear). The engine needs 87 octane and even with couple of weeks with 91/94 octane, the sound is still present.

http://www.youtube.com/watch?v=wCbi3FnuhTM

Thanks.

sounds like a timming chain issue. we have recplaced quite a few where i work at.

I already changed the 3 chains + all tensioners and the noise is still exactly the same.

When its getting cold outside, the noise cannot be heard (below 15 to 20C)

My 04 Caddy with the 3.6 V6 has the exact same symptoms, to me it sounds like the engine is pinging (pre-ignition). I wonder is it possible the ECM (engine control module) is advancing the ignition to much under the very light load, low rpm conditions? I have 77,000 miles on the clock. I'm concerned about engine damage over the long run........

Okay, I give my car to gm and they checked everything from timing chains, to tensioners, to mufflers, knock sensor, misfires, engine mount, transmission mount and nada!

Any hint?