I have 2 CrossFired cards. The fans constantly start and stop when the system is idle. Not sure if one or both are doing this. They don't seem to always do it. Disabling and then re enabling CrossFire seems to fix it, although I just tried that now and now they are just going full boar. Under load they just go full boar also with no problems.
2x Sapphire HD 3870 512 MB (ATi Radeon HD 3870)
Driver packaging version 126.96.36.19901-150113a-179335E-ATI
AMD Vision Engine Control Center version 2012.1116.1515.27190
Windows 10 Pro
MSI 970 Gaming motherboard
EVGA 750W PSU
Please don't tell me to just get new cards. I may in time. I also understand that it's probably due to the age difference between the cards and the system.
By the way my GPU meter always says 0rpm for fan speed no matter what.