do you have it

Go to Options, then the Keyboard and Mouse controls. From there, change "Allow Developer Console" to "Enabled". Afterward, you can press the ~ key (it's above Tab on most keyboards) to open the console. Then, before noclip will work, you need to type in the command "sv_cheats 1" without the quotation marks. After that you should be able to type in "noclip" and fly freely! My advice, use the command "bind v noclip". That way, you can press V to turn noclip on and off at will, like in Garry's Mod. Word of warning though, turning on cheats will disable achievements! So if you're going for those sweet 'chieves, you'll wanna finish the game without cheating first.