New Recording

Sounds like a loose heat shield to me.

I found a tiny hole in the muffler and plugged it with exhaust cement and that elliminated the sound that appeared to come from the middle of the car, Now that this sound is gone , I think I hear the sound of timing chain (metal rattle type sound) ..but this only happens when car is cold and until it warms up a bit (for a couple of minutes). I thought the chain would stretch (expand and become looser) ) as the engine heats up and the noise would get worse but noise gets better ?

Glad to hear you figured out the sound!

The timing chain doesn't stretch significantly based on temperature if it is about 2' long, it would expand about 000.0312" between 0 degrees and 200 degrees Fahrenheit. For comparison, the aluminum block and head would expand about 000.0468" along that axis with the same temperature change. Since the chain'* expansion is in two spans because it is in a loop, the difference in length from crank axis to cam axis is more like 000.0156 longer. Based on thermal expansion alone, the chain would actually end up very very slightly tighter after a temperature rise, instead of looser.

It is more likely that you are hearing cam phasers being manipulated by the PCM and/or quieting down from getting warmer oil, or possibly the cam chain tensioners tightening up as oil pressure rises.

My engine has 240,000 km. Should I assume this is normal? or should I be doing something about it (like additive to oil)?

I'm not fan of additives.

240,000km is about 149,000 miles. Yeah, it has the right to make a few noises.

I would make sure to keep the oil changed on a good schedule, and use quality oil.

I generally use 5w30 synthetic changed every 8000 km

The sound sometimes happens and sometimes doesn't ..does this make any difference ? This morning it was a bit cooler but engine started right up with no problem.

I have had engine light with 0446 code recently and since it was cleared about 500km ago and hasn't come back on.

Cool!

If it is not happening every time then it is probably not something to worry about.

P0446 is for the evaporative emission control system vent control circuit. This is not related. Glad it hasn't come back on though.

the check engine light came back 0446 was the code when last checked. Is there a common cause for this on ecotec engines that I should check?

That'* a vent code. The vent is located at the canister. You may just need to clean it out. Also a purge valve being stuck open can also cause that code.
Without a good scan tool the best thing to do is start at the vent and make sure the vent is clean.
Here'* a video on that code.