Mark - that is horrendous and if they have attempted numerous times and cannot fix that, firstly I am amazed (surely with something so obvious and easy to replicate) that they cannot fix it and secondly, I totally understand you now rejecting the car. I can confirm that I rejected my XKR for a sound much less obvious than that (and more intermittent to much harder to replicate consistently)!
I have terrible OCD when it comes to rattles and cannot stand the slightest noise so I do feel your pain. It must be especially hard when Merc aren't helping and I wish you the best of luck and I sincerely hope you get what you want out of this.
Out of interest, have you escalated this to the Merc UK's boss (known as their 'President' IIRC)? I complained directly to him once about cracked discs that they wouldn't replace on my C63 and he authorised it...I know this is something on a much larger scale but it may be worth a go? (Saying that, I can't remember his name or contact details however I did get them from someone on MBClub so may be worth asking over there).
Originally Posted by dobsy
I took the wife's out the other night and whilst there's the odd creak here and there as with any car ......
New cars are (or should be) sufficiently well engineered to ensure no unwanted noises are present. I never accept anything other than blissful silence (although this is harder to get than it should be!)