Air in water supply lines

I had a leak under the kitchen sink and had a plumber replace all the lines and shut off valves - hot, cold water, ice maker and dishwasher. He also replaces the lines with PVC. Everything was fine for about 5 days, but now everytime I turn on any faucet or the shower there is a small amount of air in the line. This occurs when no water has been run in the house for several hours. What could be causing this?