I guess it depends on the roads you drive on and the miles on your car, but I don't have any rattles yet. (has it been 2 years already? :-0 ) So I don't think it is typical of the SLK convertible.
You just need to find the rattles and get them fixed. Or maybe turn the radio up louder
I noticed the flecks in the windshield in my SLK 320. That was the most pitted windshield of any car I have ever owned. I think I am already seeing some pits on the new SLK.