Try the Talismoon Whisper Max. I fitted one to mine last week. Its a little quieter than the stock Deltas (around 2-3db) and makes a rasping sound with little mechanical clicks now and again as it uses rifle bearings I believe. Most Deltas make a very loud whining sound when the RPM increases with the heat of the GPU. Whisper Max also has 3 different colour LEDs (Blue is installed as std but you can mod to Green or Red as they include the LEDs to do this easily). Also cut away the plastic & metal fan grills at the back to increase the air flow a little (not much but every little helps!!).
Problem with 360s is that you really need a high RPM high CFM 70mm dual fan to fit into the space the stock one goes and getting it powerful enough is impossible with low noise.
I tried dual 80mm silent fans yesterday but it overheated as they do not extract the heat quickly enough as they are outside the case 4cm away from the original fans mounting point so had to put the Whisper Max back in. Putting your 360 into a desk cupboard is also possible but unless you ventilate the space with cool airflow your 360 will get hotter as it will heat up with the hot air being retained in the cupboard. I tried this as well and found you have to use some sound deadening material inside the cupboard otherwise the fan noise can sound worse as it amplifies the noise more and the fan RPM increases as it gets hotter.
Lian Li XB01 case will be almost silent as it comes with a 120mm fan and you can mod it with more fans but its around £80 + P&P so quite expensive.
