The drive cages look to be horizontal mount for the drives. What I'm speaking of is to have the drives sit on their sides, not flat. But that is a small case. Do you have a fan on the left case cover? If not then I'd put duct tape or similar product over it (on the inside of course) to prevent air being sucked in through that hole and ensure the air flow is going the direction you want it to. In fact I would try the following setup myself... Two front intake fans and one exhaust fan in top of the case or rear of the case would depend on if the CPU has a directional fan and that is where I'd put it, tape up the other air holes as they do not benefit good air flow. In fact I would only have the two front fans as intake and any other fans you add be exhaust. This will maximize the air flow across the hard drives. If you were to run all the fans at 7VDC that might be fine too but you would have to experiment. Since computer parts are still on sale, purchase a few of the 4 pin molex to 3 pin fan adapters. You can then relocate the 4 pin side to 5VDC or 7VDC.
The thing to keep in mind is look for how the air flows in your case. Plan a path for it and don't forget about all heat producing components. So you cool the drives down but now you superheat the north bridge, not good. Feel the component temps and just make a plan. In one computer I created many years ago I actually made cardboard air flow diverter to keep the hot north bridge cooler. It wasn't very large and finding a good way to mount it was the only hard thing because I wanted it easily removable and reusable. Cover up all unused holes with tape from the inside of the case. Don't use crappy tape that will fail over time. A roll of Duct tape works, I personally use Gorilla Tape, that shit is hard to take off.
Of course you might be fine with the setup you have right now. I guess the other option is to relocate the server to a place in your home where the noise would not be an issue. Mine is in the basement although it is virtually noise free. My computer room is packed enough so for me it's a space issue.