Noisey Baseboard Heating
Beside air, could anything else be causing the noise?


Depends on exactly what the noise is, but I would be on the lookout for thermal expansion.  If you have long, straight runs, the pipe can expand significantly when heat is delivered.  If there are places where the pipe binds on the supports, it can make pinging and popping noises as it alternately binds and frees itself. If it's attached to a loose metal airflow director, it can rattle.

Because this noise isn't coming from inside the pipe, pulling or shifting the pipe as you turn it on will change the sound significantly, but shouldn't make much of a difference if there is air inside.