I'll be honest in saying that this is the third out of 4 places I'm putting this, that's how desperate to fix this I am. About 4 months back or so I installed a Sound Blaster Z sound card into my PC so I could power my headphones better and power my microphone better. But recently I've had a really bad coil whine problem and I'm struggling to pinpoint the cause of it. Whenever my computer idles around and not doing anything other than gaming, it's fine. I get no whine through my headphones and the mic transmits no whine whatsoever. But as soon as I ramp my GPU up, I get some whine through my headphones and massive whine that transmits through the microphone rendering it completely useless. On-board audio doesn't have this issue, but it's not powerful enough for my headphones or mic. I'm extremely desperate now.
It's not really coil whine, it's electromagnetic interference from the GPU. Spoiler: Video about EM Interference Just do what @BlueScreen said and it should significantly reduce, if not completely solve, your problem. If the reduction isn't satisfactory, you could grab a PCI-E extension to get the sound card even farther from the GPU.
It actually does now make sense especially for the mic. The mic input to the chip is partially unshielded. It'll get new duty in the lower-most PCI-e slot tomorrow.
