Air finds its way into a water pump through many avenues, but the most common reason is improperly priming the system during installation. Another common source of vapor ending up in water lines is due to the presence of methane gas within the well itself. While not harmful at these levels, it can cause the water lines to sputter.
Sputtering water lines are probably a minor concern when a home's water smells of methane gas. When methane is in the system, the water intake line for a pump can become damaged, which leads small gaps or holes to form. This allows water to leak out and air to seep in.
Each of these scenarios is a fairly common problem that professional well technicians come across on a regular basis. When they do eventually crop up, the most appropriate solution is to simply call in a professional to both diagnose and resolve the problem. Too often, homeowners relying on well water attempt to troubleshoot and resolve the issue themselves. This can create more problems than it addresses as the average person simply is not familiar with the system itself or the intricacies of plumbing. Unfortunately, most of the pump systems that the average homeowner is familiar with are either completely different or not used for such an extensive and long term purpose.