Depends on shape and material kettle is made of.

Yes, it sounds a bit like popcorn popping, but not as loud. It happens when the metal bottom reaches the boiling point, but the water above it is still below the boiling point. Bubbles rapidly nucleate in contact with the bottom, but as they expand into the surrounding water, they cool off and shrink back to water. The implosion is so fast it creates a shock wave that reflects outward and is heard as a tiny "pop". The rumbling sound is the result of hundreds of these pops happening every second.

The reason why the internal curly element design makes more noise is because the heating element heats a smaller area of the bottom (just where the coil is). Given less contact with the water, the heated surface gets hotter quicker (before the water gets too hot to implosively condense the bubbles).

It could possibly be something to do with the thickness of the kettle or how much water is boiling

it is all about the size of the spout, lik ewhen you whistle u need 2 make a small hole wit ur lips, u cn make different pitches by shaping ur lips or blowing harder

thisbrings me on to my 2nd point, if there is a lot of surface area and a lot of steam is being generated which in turn is being forced out of the spout it can sound different accordind 2 the force/speed the steam is travelling!

hope this helps, might not be correct but im only 12!