I've just noticed, if I remove all devices apart from the clock source from the aggregate, problem goes away. As soon as I add in another interface, it's back to distortion.

Hi, this is about crackling / distortion that's encountered when using audio devices in an aggregate device. I'm guessing if you don't know what an aggregate device is, you don't have to worry. It's just a way of combining devices into one. If you're getting crackling without an aggregate set up, I'd have a look around and see if anyone else is encountering the same issue. Sorry I can't be more helpful! Hope you figure it out.