Problem: Low double-beep sound recorded in video mode while using a Nikon D810. It only seems to happen when I use my Sandisk Extreme Pro 160mb/s cards. There is a double-beep every second.
Solution: Use the slower cards.
Is there anyone else encountering this? Just wanted to warn you before you go out to record an important shoot. Makes the audio from the video useless. On the web, there seems to be other makes and models of cameras (both DSLR and video cameras) that encounter this.