Hey there, if you don't feel like changing your case (which is understandable), here is what I recommend.
If I'm not mistaken, a "Hyper 212 Evo" would fit well in your case, and improve your cooling tremendously. Likewise, the default fan on the Hyper 212 Evo is relatively quiet (much more quiet than the average graphics card fan).
If you want to keep your existing cpu cooler (assuming it's a non-stock cooler), you can always swap the fan for a "Noctua" fan that are known for being silent.