The 15 I have has two fans: one I presume for the CPU and another for the Nvidia GPU (which I don't use). However, once both have started neither shut off until the machine is suspended. There is a suggestion that this is due to a firmware bug related to suspend/wake and it doesn't happen from cold boot. I haven't tested it out, will give it a go next week.

I've also been used to MacBooks and so the XPS does fall short somewhat in comparison - minor flexing and creaking and much less heat dissipated via the chassis (plastic vs aluminium). I have an Intel i7 in the XPS that appears to consume quite a lot of power. I'd much rather change it for a less powerful, more frugal i5 but alas it is too late.