Question 1

what is the CFM for this fan?

Accepted Answer

5

Question 2

This might be a dumb question, but what is the operating voltage?  Don't see a rating anywhere in the description or reviews and hoping it's 12VDC.

Accepted Answer

12 volts DC. 3 pin connector but comes with a 2 pin adapter and extension cables.

Question 3

Have anyone used this to replace the 35 mm assist fans on an asus sabertooth z87?

Accepted Answer

I have it in my Asus Sabertooth Z170 S and it fits just fine! (just remove the two Anti-Vibration Pads in contact with the TUF Armor to be able to close the lid).
Take into consideration that in my version of the Sabertooth, the assistant fan position is vertical, inside the TUF Thermal Armor, between the IO Shield and the heatsink VRM, and works as an intake fan, to blow fresh air into the motherboard. 
Every Sabertooth Assistant Fans are designed to work differently. I believe in your motherboard the fan is place horizontally, on Top of the TUF Thermal Armor. 
Also the fans needed for your Sabertooth Z87 had a mini 3 pin connector, my motherboard use 3 or 4 pin (regular size) connectors. Therefore the original assistant fan of the Sabertooth Z87 cannot be plugged in the Assist Fan 1 connector of my Sabertooth Z170 S. I know this because I was looking for the fans for my motherboard and buy the Z87 version by mistake, couldn't use it, so I buy the 40mm Noctua, and it works great! Really improved the temperature inside my case. Also my motherboard use 40mm fans. I'm not sure of the Z87 assistant fans size, but putting the two of them together, are exactly the same size, only thing adding up to the size variation, are the Noctuas silicone anti vibrations pads, that can be removed.
Hope this helps even a little bit.

Question 4

What is the sound decible rating

Accepted Answer

http://noctua.at/en/products/fan/nf-a4x10-flx/specification

17.9

ASIN	B009NQLT0M
Air Flow Capacity	8.2 cubic meter per hour
Best Sellers Rank	#47 in Computer Case Fans
Brand Name	Noctua
Compatible Devices	Desktop
Cooling Method	Air
Customer Reviews	4.8 4.8 out of 5 stars (11,255)
Global Trade Identification Number	04716123314691, 07123290462549, 07133363436841
Item Dimensions L x W x H	1.57"L x 0.39"W x 1.57"H
Item Weight	50 g
Manufacturer	Noctua
Material Type	Fibre-glass reinforced PBT
Maximum Rotational Speed	4500 rpm
Model	NF-A4x10
Noise Level	17.9 db
Number of Items	1
Number of Packs	1
Part Number	NF-A4x10
Power Connector Type	3-Pin
UPC	163121136772 809185325669 806291857176 804066543873 842431014054 102930672934 782386476753 172302657890 132017920719 808111662861 809385174852 168141421648 851905102548
Unit Count	1 Count
Voltage	12 volts
Warranty Description	6 year manufacturer’s warranty

Noctua NF-A4x10 FLX, High-Performance Quiet Slim Fan 40mm | High Airflow Low Noise 10mm Slim Design, for PC and Electronics Cooling, 2 Speed Settings 4500/3700rpm 3-Pin

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