Recently, when my heating system starts I can hear gurgling in the base board pipes. Local plumber came and said our burner was doing fine. So what causes this?? Should we be concerned?

(Also charged 165$ for 2 men to diagnose this in less than 10 minutes.)!!
The gas fired system is about 15 years old.

Most likely air in the pipes. Bleed the lines

There should be a temperature & pressure gauge on the boiler. What is the pressure showing? For a single story & most 2 story houses, 12 psig is fine.