What's the mileage ?
Hissing sound is normal for this brake system. If you don't have any errors there's nothing to panic. Accumulator running is the standard operation to re-pressurise the system (which gets de-pressurise after few minutes from switching off the vehicle)
If you can replace brake fluid every 10-15k, that will help longevity of the ABS modulator. As for accumulator and the pump, the pump obviously will wear out over a period of time and will need replacing when it doesn't generate enough pressure or takes prolonged period to pressurise the system.