My hot water baseboard system was bleeded two weeks ago when the water flowing noise was heard.

Now, every time when heat turn on, there are several big bang noise from the pipes on my second floor. Please tell me what is going on and what I should do?

I last week, I changed new expansion tank. When bleeding the water, the pressure readout is 10-12 psi. When bleeding is finished and the water is heating, the pressure readout is 20 psi.