Strange noise at our house with a water pressure drop - what is it?

Hello all:

Several times a day for the past several months we hear a whistling sound, only a second long - almost like a bottlerocket or toy rocket is taking off. We immediately notice a second long drop in water pressure when we hear it. Happens at random times throughout the day.

Environment: Single-family standalone home, Southern California. Built in 1989 into the side of a hill. Thus, street level is the upper floor, and the lower floor is below street level. Since we assume it has something to do with water, we do have a sump pump, sewage pump, and a gas-powered water heater. Visually, we can't see any damage, leaks, or broken or non-functioning components.

What is this noise?

Thanks!