take the af 120s out, clean them properly. You can take the blades off and wash them thoroughly. add a bit of silicone/grease in the center part of the fan (moving) if you think it'll help. Move it a couple times with your fingers, sounds great again.
AF/SP 's are great fans cost/performance.
Also it's very important how all the fans work, where do they push the air. Best case scenario is to have positive pressure (? pls correct me if i'm wrong). this means you have more intake than exhaust. This will help you with the airflow and dust.
It also depends where you have the case and how obstructed the fans are. In optimal conditions i'd say AF 120 are fine. 2 front + 1 back to push air IN case, and 2 top to exhaust.
The positive pressure will make sure all the extra air will gtfo of the case through the small cracks you have in the pci/back of the case.