Is it bubbling like a freshly-opened beer? Or are there bubbles like when you aggressively pour water from the tap (i.e. just a little, no specific origin)?
Me thinks you just have a little CO2 from the fermentation and shouldn't be concerned about it. That "head" you see can just be the small amount of CO2 that is dissolved in the beer.
15 Gallons of New England Hard Cider - Four yeasts: Ale yeast, two cider yeasts, and a champagne yeast.
Helliday Ale and Chocolate Nitro Stout