Short answer: rapid cycling, where the pump starts and stops every few seconds, is nearly always a pressure tank that has lost its air charge, a faulty pressure switch, or a leak somewhere in the system. It is worth fixing quickly, because constant starting is what destroys pump motors.
Why it matters more than it sounds
A pump motor draws its highest current at the moment it starts. A pump that starts a few times an hour will last years. One that starts every few seconds is doing thousands of extra starts a day, and the motor, capacitor and switch contacts all wear out quickly. What begins as an inexpensive repair becomes a pump replacement.
The usual causes
A waterlogged pressure tank
This is the most common. The tank contains a cushion of air, usually behind a rubber bladder, that holds pressure between pump cycles. When the bladder fails or the air charge is lost, there is nothing to hold pressure, so the pump starts the moment a tap opens and stops the moment it closes. Often the tank can be recharged; if the bladder has split, it needs replacing.
A faulty pressure switch
The switch has a cut-in and cut-out pressure. When the gap between the two narrows, or the contacts pit and stick, the pump cycles. Switches are inexpensive and quick to replace.
A leak in the system
If the pump starts briefly when nobody is using water, something is leaking. It may be a dripping tap, a running toilet cistern, a leaking non-return valve or a buried pipe. A pump cycling overnight with every tap closed is the clearest sign of a leak you will get.
A failed non-return valve
This valve stops water flowing back down the line. When it fails, pressure bleeds away after the pump stops, so the pump restarts to make it up, again and again.
A blocked inlet filter
If the pump cannot draw enough water, it reaches cut-out pressure, stops, loses pressure immediately and restarts.
A quick test for a leak
Close every tap in the house, including outside taps, and make sure no appliance is running. Watch the pump for ten minutes. If it starts at all, you have a leak, not a pump problem. If it stays off, the fault is in the tank, switch or valve.
What to do now
If you cannot get to it the same day, switching the pump off at the isolator when nobody is using water will protect the motor, as long as the house is not relying on it for supply. Otherwise it is worth getting it checked quickly: the pressure tank and switch are inexpensive compared with a replacement pump.
Need a technician today?
Pump or pressure problem? A technician can usually be with you the same day. Licensed, fixed price before any work, warranty on every repair.