So, I ran a couple tests on different configs with the splitter. I connected to splitter to the MB, the 3 pin to the Noctua fan, and the 4 pin to the CPU fan. The CPU fan now runs based on the load. Speeds up under load and so on. And the Noctua fan is in sync with it. It speeds up along with th4e CPU fan. The way you originally described in the OP, the fans were on full blast 100% of the time, which was loud. Under full load running a benchmark, both set ups never got above 80c. Stock, with the fan shroud and no extra fan, it got to 81c. But I have a 1030 card showing up later this week which will add extra heat. And that is the reason I wanted the extra fan.    edit: I just ran another test and noticed the Noctua fan doesn't run at all when not under load and the CPU gets below 30c. I'm gonna leave it like this and monitor the temp for a couple days. Having both fans run on full blast 100% of the time was annoying. 
Recently ive tried upgrading my inspiron 3891 with a noctua AF-8 80mm fan, this has been a sucess.I proceeded with the upgrade by using the pwm splitters included with the box. How this works is, first I plug the 4 pin pwm splitter in the 4 pin cpu header, then I plug the noctua fan into the 4 pin part of the splitter, I plug the silent adapter in the 3 pin part of the splitter and finally connect the cpu fan into that silent adapter.After all has been connected, screw down the the 80mm noctua fan in the rear of the case using the screws or rubber. Remember the order of fan connection is important or else you could end up with only one fan working.Here are the links for the fanCanada: [URL]  [URL] [URL] to purchase the 4 pin model)[IMAGE][IMAGE]
INSPIRON 3891/3880/3881 CASE FAN UPGRADE