I just added a full bath, pex supply, PVC waste lines, using an air admittance valve to vent the wast system until I can get it out through the roof.  Bath has low flow toilet I got at a supply discounter, “gooseneck” faucet on a pedestal sink & a shower head w/ no tub fill, both of those fixtures are off the shelf from big box stores.  Also just replaced kitchen faucet with a “gooseneck” fixture from the Depot or Lowes.

Shortly after all that fixtures all through the house started to spurt air, only from the cold lines, not the hot.  Plumbing contractor checked the well pressure tank, the line between the bladder type tank and the well was pressurized and held.  There was water in the line from the submersible pump to the fitting at the top of the well casing indicating the foot valve is holding.   Still there is air getting into the lines somewhere…where else could it be coming from?  An air of mystery???