I would monitor the problem carefully. The bubble MAY be from refilling the system. If they are still present in a week or two, then I'd be more concerned.

In the early stages, a blown head gasket may only exhibit symptoms after the engine has run for a half hour or more and is heated to above the lower end of normal operating temperature. A compression check will often NOT pick this up unless run when the engine is well above the minimum operating temperature (say with the engine at 95-100 deg. C.). I had OM621 and 61x engines that despite being driven with a light load a half hour to 45 minutes at a time, remained in that state for years, only requiring that perhaps a half pint or so of coolant be added every 3-6 months. They could not be counted on to endure several hours of driving under heavy loads or in really hot weather. A new head gasket solved the problem. The "track" on one of the head gaskets (I didn't examine the other one) was barely visible when examined after the fact. It was little more than 0.005-0.010 mm in width.

Marshall