It was probably coil wine.
Most GTX 1080 a very noisy when first purchased. It will dissipate over time. Mine still makes some noise during very particular situations - namely bench marking, something about it makes it whine.
In gaming, even under 100% load, i don't believe i have heard coil whine in months. Just for the first few gaming sessions could i hear it.
It is common among GTX 1080s.