HVAC stands for Heating, Ventilation, and Air-Conditioning. HVAC systems include central heating and air-conditioning with warm or cool air ducted to the classrooms or window, wall, and ceiling mounted air-conditioners and heat pumps. Ducted systems may have a loud rumble as the ducts transmit noise from the central fan and turbulence in the ducts to the classroom. Window, wall, and ceiling mounted systems often have fan noise and compressor noise that is in close proximity to a number of students.
These problems are totally unnecessary. There are quieter window, wall, and ceiling units (although they are still rare). Noise in a ducted system results from poor design and inattention to noise issues.
See How to test noise levels in an empty classroom? for information on how to test if existing noise levels from HVAC are a problem.