We have a new Price Pfister single handle with spray kitchen faucet that makes a moaning sound when the faucet is on. What is causing this noise? It it driving us crazy!

There are several things it could be, one is that the supply stop under the sink could have a loose washer and is vibrating when the water is on. It also could be the valve that diverts the water to the spray.
There is also a chance that the noise is not coming from the faucet itself but maybe from a commode valve that is not turning off good and when you open the sink faucet it makes the commode valve vibrate.