I had that combo but I rarely use video on my 70d. I had the 18-135stm as a kit lens with my original 650d but I've since sold the two.
The AF is practically silent even with the onboard mic and you can only hear tiny bits of it if the room/space you're in is silent.
There are a few other STM options now (40mm, 17-55mm and that 50-250mm) and I still have the 40mm.