It could be that one or both of the fans aren't quite right, or maybe it's just the way they are. You could try moving the cables a bit further away and see how it goes, as sometimes the disruption in air flow can cause fluttering noises like this.
How did you spin them up? Was it a utility like AfterBurner to manually spin them up? If so, you might be spinning them faster than they would ever go in practice when playing a game so the problem would be moot. For example, my Palit GTX 1080 remains barely audible (see TPU's review) when running a game, or even something really intensive like Furmark designed to heat it up. However, manually spin them up with AfterBurner and the card is now not quiet at all. However, it's an irrelevant test, not a reflection of its true performance, because normal use would never cause them to spin that fast.