2006 Engine rattle-need help (with video)
 

I have a 2006 SL that just passed 100k miles. I am getting a rattling sound at idle, but also when accelerating. It doesn't go away after I have driven for a long time (always there).

My dealer has mentioned the following problems: bad left front motor mount, bad front struts, leaky AC hose, and is also recommending do the dreaded timing chain/tensioner replacement. All told about $2500 in repairs. I really want to know whether this rattling sound is due to the timing chain which I really don't want to break. The funny thing is the car never made that whining sound typical of timing chain problems.

Here is a video of the sound:

Yeah, you're due, bro. Check this video.

The thing is that is doesn't go away even after I've driven for an hour or so. Could it just be valve noise? or rattle caused by the bad engine mount?

Well, possibly. What type of engine oil are you running?

Castrol 5W30 I believe - fully synthetic - every single time (every 6000 km)

Yeah, I am also running Castrol Edge and I have noticed the engine to be a little noisier. Some say that some engines are a little louder due to using synthetic oil, in particular Mobil 1. I am going to go back to Pennzoil Platinum on my next oil change to see if I notice any difference in the noise level of my engine. That might also be something for you to consider as well.

It was the timing chain
 

Just had it fixed at the dealership and the noise went away. The tensioner was badly broken. Cost $1100 plus tax ...