I was just toying with the idea of possibly going SLI for the first time, and buying another GTX 970 G1 Gaming since I have been so impressed with it. However, when reading on Newegg's reviews (Specifically all the lower rated ones) everybody is complaining about coil whine.... and the people complaining about the noises it makes...
I have not had a SINGLE problem with this GTX 970... So it leads me to believe those reviews are more of a system issue on their parts and they are just blaming the card, when possibly they may not be able to support it or something. (and it could also be faulty cards of course)
Anyway, only issue I am seeing with going SLI on my rig is the space in the R300 I have, with the extended back plate/outer case on the G1 Gaming, I would have to modify the Hard drive rack to fit it in, and maybe even do some other modifying since the 3rd PCIx3.0 slot is so near to the PSU and this card is so damn big. Most likely not gonna be worth it though. Just thinking about it.
Anyway, like I said, I have not noticed any noises louder than my already quiet fans in my system. Then again, I don't even pay attention to it to much with a head set on. Seems like its a hit or miss thing if it does not have to do with peoples systems supporting it.