Why did Joshua think there was war in the camp?

Joshua heard the loud noise of the people shouting during their celebration of the golden calf. Because it sounded like chaos and screaming, he assumed the camp was under attack, with people yelling and running for their lives. Moses corrected him, saying it was not the cry of the overcome but the noise of singing.

Les explains that Joshua heard the noise of millions of people having a celebration in the worship of the golden calf. The shouting sounded to him like war, so he thought the people were being attacked and were probably screaming and yelling, running for their lives. Moses told him, 'No, wait a minute, Joshua, listen. That's not the sound or the voice of them that cry for being overcome, but I hear the noise of them that sing.' When Moses came near the camp, he saw the calf and the dancing; his anger waxed hot, and he cast the tables of the Ten Commandments out of his hands and broke them beneath the mount.

Exodus 32:17

And when Joshua heard the noise of the people as they shouted, he said unto Moses, There is a noise of war in the camp.

What were the Israelites doing when Joshua heard the noise from the camp?

When Joshua heard the noise from the camp, the Israelites were shouting and singing in a celebration of worship around the golden calf. They were not being attacked; Moses corrected Joshua, saying the sound was not cries of defeat but the noise of singing, accompanied by dancing before the calf.