Windows sleep mode, fan issue

I bought an AVA computer with  a Z971-Plus Asus MB. When the computer goes into Sleep mode the fan on the back continues to run. There is a wrinkle, if I manually put the computer in sleep mode, the fan stops. It is only when I let the computer timeout into sleep mode that the fan keeps running. 
 
This happens under both Windows 7 and Windows 10.
I have just installed the latest Bios from Asus 
I looked on the web, and found similar issues, but none of them seem to fit the bill.
 
Any ideas? Thx.