Giant Defy Cable Routing is pretty unremarkable as it turns out. See a picture below...external and not inside the top and down tube cable routing like many modern bikes. Generally external cable routed bikes like the Defy tend to be less creaky.

So what to do?

Easy approach is to heed the excellent advice from igosolo with a further suggestion. Shift the bike into the big cog in back and then upshift the bike with the rear wheel static which will create a very slack bare cable. Creaking almost always occurs where the arrow points in the picture below. With slack cable pull the ferrule out of the cable stop where it attaches to the front of the bike. The ferrule becomes contaminated and sticky and creates creaking. If you can find a service manual PDF on line...you will learn what kind of ferrules are used...or by creating a slack cable. If ferrules used have a sheathing extension or tongue..when the ferrule sheathing wears, there can be bare cable on barrel adjuster contact without the buffer of the sheathing which can cause a creak. For example this is very common Specialized bikes with internally routed cables. No or worn sheathing creates metal on metal contact and these sheaths tend to be not only sacrificial but wear relatively rapidly unfortunately. I have experienced this several times on my Specialized bikes. Also this ferrule design is used on DA9000 shifters and again if the sheathing wears, noise, wear and even cable binding can result. See below pic on DA 9000 which may or may not emulate what you have on your 105 bike.

A finer point. If your bike requires the sheathing and you have an aftermarket Jagwire cableset and use Jagwire ferrules without sheathing, this can easily be your issue. So find out the factory spec...look up the PDF for Shimano 105 goupset installation on Shimano's website...based upon your model year as new 105 is revised 11s and closer to Ultegra and DA 11s in design...then compare it to the Jagwire ferrules that are being used with your installation. No amount of cleaning will resolve not having the proper ferrules for a given design.