OK yeah thanks all.
I have actually only noticed this once yesterday so I will keep monitoring it. It is a vented drain for the washing machine and just washed some clothes an hour or two before that. It runs every other day too so it must have just been a "strong" one.
I'm about to have a plumber install a water softener and he will be using this same vent pipe in the attic to (acurately) drain into. So I just wanted to make sure there wasn't any other issues before I start this softener project which will be using this drain.
Theoretically, based on what youv'e said, it may actually help my situation since water will be running down the drain once a week from a much higher point of the drain pipe.