Same thing happened to me on my replacement. It may just be a universal hardware issue on all phone? Ive been trying to see if there are any 7plus owners who do not have that static or hissing with video recording. But maybe it is universal across the board.
Not sure its on ALL phones but it is on a large number of them. I had this issue and returned it as of now. I didn't want to opt for a replacement now because all replacements offered were built during the same weeks as all the ones with issues. If there is any hope, its best to place an order now and wait the 2-3 week time window in hopes something may have been addressed. I am not getting my hopes up but continuing to swap phones manufactured the same time as the ones people currently have just doesn't make much sense.