Sorry for the English...
The fluctuation of the pressure with the temperature of the brine circuit is completely normal. This is due to natural expansion and contraction.
The only way to not have this pressure yo-yo is to fit a real fairly large expansion tank with a membrane, but there is not much point, because it works also fine without.
However, the sounds in the system are not normal, and this is because you have underpressure in the system.
The installation manual for your pump (and also for Bosch Compress 6000 series) states, that you must have 2.5-3 bar pressure in the system.
When the brine is cold and pump is working, it is good if you have at least 2 bar. But you have only 0.4 bar, this is why you can have sounds, also the efficiency of the brine pump can be lower at such low pressure.
Here is the cutout from the manual for your heat pump:

You need a simple filling tool with hose and hand pump, they can be rented, and you need to add some coolant (make sure you know which coolant is used, e.g. 30% ethanol or glycol or some other), or call the technician to do it. Probably not the best idea to put 2.5 bar after the brine has cooled, because in the summer you can have then over 3 bar pressure, but 0.4 is too low.