There are a number of reasons:
The hot water has a lower density than cold water
Hot water will often be filled with tiny air bubbles, giving it a milky appearance. When old water, with lots of dissolved air in it, is heated, its ability to hold dissolved air is reduced. The air is forced out of solution and into tiny bubbles.
But most important, hot water has a much lower viscosity than cold water. The viscosity of water at 70 degrees Celsius is about $\frac13$ the viscosity at 10 degrees Celsius. The hot water will flow more quickly through the pipes and splash differently when it hits a surface.
All of these will change the sort of sound the ater makes in flowing through the pipes, and outside the pipes as well.