SLK250 rattle and vibration

A few weeks after buying a new Mercedes-Benz SLK250 in 2013 a vibration/rattle would happen while I was driving. It was intermittent, but I noticed that it would happen around 2200rpm and at varying range of speeds, 40, 65 or 110km/h, and it would last a few seconds. The dealer couldn't replicate the noise, as the conditions around the city in peak hour were not the same as when I would've heard the noise. I left it with them on another occasion and they said they drove it but still couldn't replicate the noise. Basically, they didn't believe me there was a noise.

I left the car with them on another occasion and they said they drove the car for two hours and couldn't replicate the noise. However, when I picked up the car there was only an additional 15 km on the odometer from when I left it with them in the morning. The noise still happened and over time got louder and longer in duration. On another occasion I took it to them and a technician sat with me in the car and briefly heard the noise. The service agent said to me 'well if it is a problem, it will get worse over time.' It did. Sometimes the noise lasts for about 20 seconds and is very annoying.

I went back to them a few months later when the noise was more frequent and had another technician drive with me. He heard it. They decided that the noise was caused by the vacuum pump and replaced it, but after about two weeks I faintly heard the noise again. Again, it got louder and longer over a few weeks. I took it back to them and this time they decided it had something to do with the air-conditioning and they replaced more parts. As soon as I drove out of the dealership I heard the noise again.

I took it back again and they replaced more parts. Now the noise is happening again. I took it back to them a few days ago, and the technician said that three other similar models have the same problem and that the noise is 'standard' and ' normal' for this car. The service manager also gave me that same spiel. What a load of rubbish! I've never had a car with a rattle. If had been told that this car would rattle under certain conditions I definitely would not have spent about $100,000 on it. The service manager wanted to close the matter but I said I would not accept his conclusion. I do not accept that Mercedes-Benz would design a car with this characteristic. I feel that they don't know or want to fix it and are trying to fob me off. After much insistence on my part that I would not accept their conclusion, they have said they will look into it further, but I fear they will wait until the warranty expires and then come up with an expensive fix option. I feel powerless as one person against a large company. This is my first ever Merc and I was so happy at the time to be fortunate enough to be able to afford this car. I would appreciate your help.