Great catch. That's why asked about exhaust. I didn't think the air pump leaking could be that loud. But I've never disconnected mine while it was running.It sounds horrible at the end. But what you say makes perfect sense. I hope that's all it is for his sake.Sounds like the plastic pressure hose from the AIR pump to the diverter valve is broken. The noise you hear is the exhaust pushing back into the valve.
It is possible that the hose was broken or not clipped on to the diverter valve in the process of replacing the water pump/drive belt. The pressure hose must be moved slightly to replace the water pump.
If you remove the right air intake tube and front engine design cover, look straight down under the vacuum pump and you will see the plastic tube. It is "Y" shaped and goes left to right.