If it is not the fan, then it is coil whine (which is the same noise you can sometimes hear monitors/TVs making), as there are no other moving parts. Coil whine is basically caused by inductors, which are coils of wire which electricity passes through, the coil shape causes changes in the voltage to be magnetically dampened (much like a shock in a car), these are often used in power supplies to smooth AC voltage into DC voltage.
However, the coil also experiences some physical force, and if they are a little loose, they can make a high pitched whining noise as they vibrate in place at certain power levels.
It's completely harmless, and it's more likely that the noise is coming from your power supply, but only when your GPU is really using a lot of power. If you want to be cautious, you can double check that your PSU 12V current source is sufficient for your GPU at max load, however coil whine can really vary wildly along different load levels (sometimes you will see it at low load levels, sometimes high, or medium).