That humming is electricity flowing through the heating elements causing them to heat up.
Yes, the tankless water heaters do cost more than the normal water heaters because tankless water heaters are designed to supply hot water only when needed unlike normal ones.
American Water Heaters.
American Water Heaters
Electric water heaters have not carbon monoxide.
You can buy water heaters from any home hardware store.
GE hot water heaters are made by Rheem.
Rheem provide a series of high efficiency gas water heaters. Several different companies also provide tank-less water heaters, which are very efficient, as well as solar water heaters.
"Rheem does manufacture both gas and electric water heaters. Rheem currently offers 18 different varieties of gas water heaters and 9 different varieties of electric water heaters. Both types of Rheem water heaters are known for their rigorous standards, reliability, and efficiency."
Water heaters can range from 1 unit to 10 units.
Some of the water heaters uses magnets. Magnets softens the water.
Instant water heaters, commonly known as on demand or tankless water heaters are available at large home improvement stores such as Lowes or Home Depot, or wherever water heaters are sold.
Tankless water heaters are available at most hardware/construction retail stores or online stores. Tankless water heater are much more expensive than regular water heaters.