Classic 3am nonsense, welcome to the club mate. That specific hour really does seem to have its own agenda doesn't it.
I've had similar in my old place in Telford - heavy dragging sound, completely rhythmic, stopped the second I went to investigate. Got my recorder up there eventually and picked up what sounded like laboured breathing on playback, nothing on the night itself though. Draw your own conclusions.
Honestly the attic is one of those spaces where your brain will manufacture an explanation before you've even got your dressing gown on. Could be thermal expansion, could be an animal, could be something considerably weirder. Did it happen more than once? That's usually where it gets interesting - single events can be written off, patterns are harder to ignore.
What does your attic look like, is it boarded out or just rafters? And more importantly did you actually go up and check?