Theres are things to consider before lowering the CPU fan speed; you would need to stress test yuour cpu first to make sure your cpu has good temps when under load, only then is it safe to lower the fan speed. HOWEVER do not lower it too much or your cpu will overheat and cause many many problems. Youd then need to stress test it again afterwards for a little while to make sure the temps it reaches are still good after lowering the CPU fan speed.
A very good CPU stress testing software: Prime 95
A very good temperature monitoring software: HWMonitor
In short: if you dont want to go through all that trouble, then it isnt safe to lower. You can lower it but it is strongly recommended that you be cautious.
I know sometimes CPU fans can get loud, but the CPU fan is meant to cool your CPU, if it is not doing a good job there can be serious consequences.
Hope it helps, good luck.