Bottom line is that the water system is SUPPOSED to be a sealed system, and it SHOULDN'T leak down and cause the occasional "bump". However, the system ages, valves age or get worn, fitting loosen...eventually, most will give an occasional burp now and then, and as long as it's not an actual external leak, then there is no problem and it can be ignored.
But the important thing is to check your plumbing for leaks BEFORE deciding to ignore it. Check lines, under cabinets, at joints, at fittings, at the toilet, etc. If there is no external leak, then go ahead and ignore it or turn the pump off when not using it. But water leaks, even small ones inside cabinets or under drawers, can cause alot of damage if left unchecked. Better safe than sorry.
Mine burps perhaps once every several hours, and I've determined that it is the pump leaking back into the tank slowly. But occasionally, I will notice an increased frequency; that's when I start looking, and invariably find a fitting somewhere has loosened up.
