Traveller

It sound like some sort rotating machine, like when a computer hard drive starts up but on a much larger scale.
it gives the feeling that the rest of the album with its journey of pain, beauty, struggle, patience and awe is implied. like once the machine is switched on you just know that you're in for something BIG and it can't be stopped until it resolves itself,
And then after that sound when the drums and bass kick in,
Oh My God!