It doesn't sound serious at the moment, so don't worry.
It could be an airlock, try squeezing the coolant hoses once the system is warmed up. There may be a bleed valve somewhere too, but maybe another member that knows your model can help there.
Changing the thermostat should not be expensive and will at least rule that out. If you want, flush out the coolant at the same time and refill with properly mixed 50/50 fresh stuff.
Do you do your own repairs? If not, then this will be a simple job for a garage to do for you.