Yesterday while filling a large container from a garden hose I noticed that periodically some air bubbles would emerge from the hose.
The hose is plumbed into a tee close to the main water supply in the basement - which is from our well pump. Down in the basement I could hear the bubbles proceeding through the pipe. It seemed that pretty much they would occur whenever the pressure switch turned the well pump on.
All the pipework from main supply line to the hose is visible - I couldn't detect any leaks at all. So its not like air is entering that line somewhere.
I have no idea how long this has been going on. I only noticed as mentioned because I was observing the hose for a while filling a large container, but doing this is out of the ordinary.
I can think of these possibilities:
Well running dry. Except that it hasn't: water is plentiful. It has never run dry in the past. (~9 years in this house)
Leaky underground pipe in between the pump and the house. Pump stops, some air creeps in; pump starts up, pushes air out. The pipe is galvanized steel where it comes through the wall, not sure of its composition anywhere else. But if its original (from 1973) galvanized all the way seems likely?
There could obviously be something I'm not thinking of. Any tips on how to diagnose this? At least to narrow it down?