If you don’t want to complicate, maybe putting a resistance between the mobo fan connector and the OEM fan could be the solution.
PS: While I was testing the fans (OEM fan and Noctua fan), I realized that the noise problem was related with the speed of the fan but also with the finish of the cooler’s layers. The cut of the layers aren’t rounded and I think it make noise when the air intends flow among the layers. When you separate al little bit the fan out of the cooler hole, the noise decrease a lot.