Set up your audio-recording equipment near the stove in your kitchen. Make sure all cords are secured and equipment is placed far from the heat source.
Fill the teapot with the appropriate amount of water for the teapot. Do not overfill the teapot.
Place the teapot on the stove and heat on low. Turning the heat on too high can burn your teapot or cause it to warp, so most teapots work best when brought to a boil slowly.
Record the whistling sounds coming from the teapot. Keep the microphone a safe distance from the whistling spout. Experiment with raising and lowering the heat of the stove to see what it does to the sound of the whistle. Turn the stove off, then back on again to start and stop the whistle. Be careful of the hot steam and boiling water. You may be happy with the sounds you record directly from the teapot. If not, you can edit them using sound-editing software.
Load the sound into computer sound-editing software. Audacity is a free, open-source sound-editing program you can use to edit your files. Use the sound-editing tools to trim the sound and isolate the sounds you liked best. If you have editing software with a pitch-shifting function, try shifting the pitch of the whistle to get a different tone.
Experiment with the volume controls on your editing software to see if you can make it sound like the train is moving toward and away from you by adjusting the volume of the whistle.