I want to put 18" tyres on my 2008 C Class

yes
but to remain legal you will have to drop the profile to around 265/35r18
the legal requirement is that the ride height of the vehicle has to remain the same with the bigger rim diameter so to get the side wall height ( profile ) has to be reduced
talk with tyre shops as they have the legal requirements and what rims and tyre combinations will be legal

NO you can only put 17 in tires on 17 in rims and 18 in tires on 18 in rims ,

The wheels could have been changed on the car from the standard original factory set and hence the tyre size may not be the standard factory original size.
The size of the tyres on the vehicle now is moulded into the side wall of the tyres and is easy to read.
Check all 4 tyres because larger size tyres may have been fitted as a pair to the rear.

The size might read - for example 245 45 18 or 235 45 17 (there are a huge number of sizes for each model of tyre).

The first 3 numbers are the width of the tyres in mm eg 225 mmm or 235 mm and so on. The second 2 numbers are the tyre profile size or ratio (which can be 70 65 60 55 50 45 40 and so on. The third number is the wheel diameter size. If you have 18 inch wheels on the car the tyres will read 18 or if they are 17 inch wheels the tyres will read 17 and so on.

could be touching something

the main thing is that you know how much difference is what the speedometer is reading & what your actual speed is, because you have gone from a 65 down to 60 tyre the rolling diameter is smaller therefor there will be a difference so therefor the speedometer is reading faster than you are actually going hence more fuel used over the same distance.....hope this helps......cheers.

IF THE SOUND GOES HIGHER-LOWER WHEN TURNING JUST A LITTLE ON THE STEERING WHEEL, IT WOULD BE A BAD BEARING ,SOME TIMES THE PATTERN OF THE TYRES MAKES A LOT OFF SOUND,THE WINDOW PROBLEM IS COMMON , PLASTIC WHEELS FOR THE WIRE THAT MOVE THE WINDOW GETS WORN-CRACKS, CHANGE THE WIRES ,THERE IS A REPAIR KIT AT VW.WATER LEAK CAN BE HARD TO DETECT,WOULD START BY CHECKING THE RUBBER SEALS FOR DAMAGE.

2008 BMW 323i 4 door sedan - factory wheel rim size

Rim size (standard factory spec):
16 inch by 7 inches wide.
Front and rear are the same size.

(It is possible to fit 17 or 18 inch rims to the car if you wish together with an appropriate lower profile tyre ).

(please note that different models in the 2008 3 series BMW have different size rims. For example, the 328 and higher level models runs larger rim sizes).

Hope this helps. Cheers

Take it to the dealer, any faults should be repaired under warranty with such a new vehicle. One thing to look out for: Have you had a puncture recently and had it repaired? Or any tyre work done? The sensors are located inside the rim and can easily be damaged by someone removing or refitting tyres carelessly. If the sensor is damaged in this manner there will be no warranty coverage, but you might be able to get the tyre company to pay for it (if you can prove they damaged it).

Take it to the dealer, any faults should be repaired under warranty with such a new vehicle. One thing to look out for: Have you had a puncture recently and had it repaired? Or any tyre work done? The sensors are located inside the rim and can easily be damaged by someone removing or refitting tyres carelessly. If the sensor is damaged in this manner there will be no warranty coverage, but you might be able to get the tyre company to pay for it (if you can prove they damaged it).

18" 225/40's on the front and 18" 255/35's on the rear