My power shower makes a noise. Is it the motor/pump? It always happens when the shower hasn't been used for a few hours. The noise lasts for 10 seconds to a minute. There is air in the pipes/system, which can be heard coming out the shower head in the video. I've tried to run the water to get rid of an airlock if there is one, but when I do that with the shower pump on full it loses pressure, and doesn't seem to clear the air. It's an old system, hot water tank and pipes.