Boiler losing pressure overnight
Low Pressure

It is normal for the boiler pressure to rise when the internal water is hot and fall when it cools. However a pressure close to 0 is a concern.

https://www.britishgas.co.uk/home-services/boilers-and-heating/guides/boiler-pressure.html

Topping up the boiler is fairly easy. There are many online guides and videos showing you how. Check your boiler's manual to find the correct filling pressure. This should be measured when the boiler and radiators are cold.

If you keep having to top up the boiler to maintain the cold pressure, then it is likely you have a leak or water is being lost through the pressure relief. Repeated topping up will dilute your corrosion inhibitor and it will become less effective.

Bleeding air off either manually or through an auto vent can also reduce the pressure. This is common after a major refill, but will reduce over time.

If the cold pressure is correct and the hot pressure is too high then the expansion vessel might be faulty.

A faulty pressure gauge could also give incorrect readings.

Rattling Boxes

Without a photo, I don't know for sure what these boxes are, but they might be zone control valves that switch your downstairs/upstairs radiator circuits on/off.

Normally they are pretty quiet. Perhaps you have some air bubbles trapped in the circulating flow. These can cause noises when passing through tight spots like a pump or a valve. This might sound like a light fizzing or a kind of tapping noise.

If you are concerned then get a second opinion from another professional.