They would hear the person being shocked plead to be released, bang on the wall, or even complain about a heart condition, and finally be silent. 65% of the participants delivered the maximum shock.
They usually became very agitated or angry with the "professor" conducting the experiment, but continued anyway. The "professor" would tell them that it was painful but safe and urge them on. If they refused, he would finally say "You have no other choice, you must go on." They obeyed the authority figure. The fact that the experiment was sponsored by Yale also added to their willingness to continue.
Before participating in the experiment, when people are asked "If a person in a position of authority ordered you to deliver a 400-volt electrical shock to another person, would you follow orders? Most people answer like I would, with an adamant no!
When asked how many people would deliver the maximum shock, Yale students responded like I would have with 3%.
I'm sure I would not deliver more than 30 volts no matter what the "professor" said to me. Of course I would never find out because as soon as it was revealed to me that I had to shock someone, I would say no, and leave.