Check and top-up the pressure on your potable water expansion vessel (a large white vessel out of your shot). There will be a car tyre schraeder valve at one end, where you can attach a tyre pump.
1. Turn off the cold supply.
2. Open any hot tap.
3. Pump the air pressure (you may find it at zero) up to the value indicated by the manufacturer, usually around 3 bar. As you pump, water will be expelled from the open tap.
4. Close the tap, remove the pump, turn on the cold supply, enjoy.