It depends on the type of car. It could be a wheel bearing, differential (if it's rear wheel drive) cupped tires or possibly a drive shaft carrier bearing or u-joint (again, assuming rear wheel drive)
the spare tyre.
It's almost always a wheel bearing that would cause that noise.
I read that bad motor mounts can cause this knocking sound
Usually this is classified as a "growl" in terms of a dog. The sound can be released from the back of the throat of the dog, which emits a low, rumbling noise that usually means contentment, happiness, and/or comfort. dogs do this in prepreation to bite.
More than likely it would be a dying hard drive. Back you data up asap and run a check through the BIOS.
An echo is a sound that bounces back to you after reflecting off a surface.
If you performed this task with the computer running (power on), you may have shorted out the sound card. Always, when installing or removing any electrical/sound connection from the back of a PC, turn it off first.
on cars that run with fuel injection it would be the fuel pump, or high performance cars that use electric fuel pumps.
That would be the sound of your mother-in-law.
That would likely be a mechanical or longitudinal wave, where the particles of the medium oscillate back and forth along the direction of wave propagation, creating the motion. Examples include sound waves and seismic waves.
A horn type instrument called the vuvuzela.
As sound waves travel from air to a solid, they would typically become faster and more intense due to the higher density and speed of sound in solids compared to air. This transition can cause some of the sound wave to reflect back into the original medium and some to transmit into the solid, leading to changes in the wave's amplitude and frequency.