OK guys, I know I'm quite ignorant in photography, so I'd appreciate your help with my issue. I was taking some pictures of my friend, who was sitting on the sofa next to the Christmas tree with some lightning. So, when I was far away (~1 foot) from the tree, flash was working just fine. However, when I was approaching the tree (~ few inches) and pressing the shutter button half-way, the flash got active for 1-2 seconds. I could literally see the light during these 1-2 seconds and it was making some noise (like the one you can sometimes hear from light bulbs). I was not touching the lights with my camera/flash. However, when the Christmas tree lights were off, everything was fine, even when I was close to them. Any thoughts on why it could happen? Is there something wrong with what I was doing or with the flash itself? I was using Canon 5d with 600 EX-RT flash.