Rob, although not in the car (if you use the recirc function on your climate then this stops the fumes from entering), if you have a sniff under the bonnet behind the engine, you will smell if its cracked..
Well I might have been paranoid with my new car, it does make the diesel clatter sometimes but normally it's pretty quiet. It's not petrol quiet (apart from when moving) but it definitely doesn't make the normal diesel clatter. I'm not saying you could put it next to a petrol and not tell the difference or anything, it's just the quietest diesel I've ever been in.