I do not know the answer to all your questions.  I think I know some of them.

the middle front fan is reserved for a possible liquid cooled gpu.  traditionally the lower front fan is a PCI express fan for air cooled gpu. both the middle and lower front fans can be used for liquid cooled gpu. https://images.hothardware.com/contentimages/article/2757/content/small_Alienware-Area-51-R5-Full-Interior.jpg

you can add a middle front fan if you plan to do high power SLI or corssfire dual to triple gpu.

I believe Dell uses all 120 mm fans for the chassis.  They are all 4 pin PWM fans.  however Dell likes to use high current servers fans (Nidec).  Sometimes aftermkt fans that do not draw much current may generate an error on start up but would otherwise run well.  I think the Alienmasters in this community have often recommended the Corsair ML120 Pro which does not generate BIOS startup error in Alienware.

the top rear fan is always for radiator fan of cpu AIO cooler.

the top front fan is always called a memory fan as in Area 51 R2.

one of the 5 fans you see is likely the liquid pump rpm for the cpu AIO cooler.  It should always be fixed at 100%. so you have can 4 physical chassis fans plus one "cpu fan" which is the pump speed (no physical rotating fan blades).

Yeah, I would like to move to liquid cooled GPU when the next Gen comes out. So I only have 3 fans right now (not including the GPU blower) that I can replace then right? The 2 front fans and the rear upper aio fan? Why does hwinfo show 6 fans all at varying rpm levels? I guess there is the PSU fan and that might be 1 of them but that still leaves me at 5.

Unrelated question.. Can I use any other PSU in this other than the Dell 1500w?

any good name brand psu would do.

you have two fans at front, two fans at back (rad fan+liquid pump), one gpu fan, that is 5 fans. not sure where you would see a 6th fan because you said you do not have front middle fan installed. I do not think system would detect psu fan because that is totally out of equation as far as AWCC knows.

AWCC doesn't see the 6 fans, it just shows the 5 you mentioned. Hwinfo shows 6 though and that's what threw me off. It shows fan-0 through fan-6. Thanks. 

Ah now I understand you were curious of HW.  Well if you look at motherboard of R5 I can count 5 fan headers (#1, 32, 30, 29, 16) plus the gpu fan, that would be 6.

• processor liquid cooling-assembly fan (LCM_FAN1)  aka rad fan
• processor liquid-cooling assembly pump-fan connector (PUMP_FAN1)  aka liquid pump rpm
• top fan connector (TOP_FAN1)  aka memory fan
• processor liquid-cooling assembly fan connector (MID_FAN1)  mid front fan
• PCI-Express fan 1 connector (PCI_FAN1)  aka PCI front fan

that makes my head spin when I read these AW header names.

https://topics-cdn.dell.com/AlienwareArea51R5_ServiceManual/images/GUID-D086194C-BBE3-43D8-9452-810BE957E645-low.jpg

Yeah right. Come on Dell, how about just labeling them fan 1, fan 2..etc.Anyway, still confuses me a bit because I have no middle front case fan yet all of the fan labels in hwinfo (fan 0 through fan 6) are showing an active reading. Thanks for everyone's input. 

I have to say I am interested in being a detective for missing alien fan in Area 51.

I got an answer for you.

there are actually two U.2 fans built in two 2.5-inch drive bays for SATA or U.2 ssd (hidden on the dark side of moon).  They are tiny but noisy like puppies.  DP/N XMH25 https://img2.parts-people.com/products/XMH25a1M.JPG

As A51 R1 has hard drive fan on the other side, so does triad 2.0. speaking like a paleontologist studying alien dinosaur fossils knowing Dell evolution design has always built new model borrowing concept from old.

In your 6 fans, you actually do not see fan3 aka aux fan or middle front fan. makes sense. you don’t have it installed yet. Interested in 7 fans?

fan 0,1,2 are aka LCM fan1, top fan1, PCI fan1=12cm rad fan, top fan, PCI fan.

fan 4,6 at 2700-2800 rpm are aka U.2 fan1 and 2.  small fans need to spin faster.

fan5 at 5000+ rpm is the AIO liquid pump rpm, same as that in Aurora.  It is not a real fan. Of course that 5k rpm had made some user quite concerned something wrong.

PS the 6-7 “chassis fans” do not include gpu fan or psu fan

Thank you for your help detective. I appreciate it. 

try disconnect the two U.2 fans and see if that reduces noise significantly.

"aftermkt fans that do not draw much current may generate an error on start up "

No the issue is the wiring. On some models they make the wires all black so you cant see what they are doing.

Regular fans on Dell sometimes require re wiring.

Dell sometimes swaps the wires around so that you will fry a dell fan on standard and the other way round. this is true for 3 wire as well as 4 wire fans.

some models have 5 pins but only 4 wires are used.

Dell does not publish this. And some dells use standard wiring.

I believe this is correct.

YMMV

4pin PWM on 3 pin

DELL FAN WIRES AND COLORS

https://www.amazon.com/Alloet-Connector-Adapter-Converter-Extension/dp/B07HF7YZ5W