You can separate these by ear and by timing before anyone gets a meter out, and the three sounds you describe are three different things.
"The whole unit changing its mind, loudly, for a minute or two" is almost certainly a defrost cycle, and it is normal. In cold damp weather the outdoor coil ices up, and the unit reverses itself to melt it. You hear the reversing valve change over with a distinct clunk, the fan stops, the compressor note changes, and there is often a hiss and some steam. It lasts a few minutes, happens periodically in the conditions that cause icing, and then everything returns to normal. If that is what you are hearing, it is the machine working.
What is worth checking is how often. Defrosting every couple of hours in genuinely icy weather is expected. Defrosting every twenty minutes in mild damp weather suggests poor airflow across the coil, a sensor problem, or an installation too close to a wall — and that is a real conversation to have.
The low rumble that comes and goes is the compressor, and here is the thing about the number on the datasheet: a modulating unit runs at whatever output the heat demand asks for, so its noise varies continuously. The quoted figure is at one specified operating point, usually a mild one. It is not a promise about a cold night at full output.
More usefully: the datasheet almost certainly quotes sound power, and what you hear is sound pressure. They are different quantities and the difference is large — power is a property of the source, pressure is what arrives at your ear and falls off with distance. Comparing the datasheet number to a phone app reading and concluding you have been misled is the single most common mistake here. Compare like with like or the comparison means nothing.
The fan is the one you can usually do something about, and it is where I would start:
- Most units have a quiet or night mode that caps fan speed and compressor output during set hours, at the cost of some capacity. Frequently not enabled by the installer. Ask.
- Airflow obstruction makes fans loud. There is a minimum clearance in front of and around the unit; check the actual installation against it.
And the one genuinely worth pressing on: mounting. A compressor sitting on a rigid bracket bolted to a house wall turns the wall into a soundboard, and structure-borne noise into a bedroom is not fixable by anything you do outdoors. Anti-vibration mounts, and preferably a ground-standing frame rather than a wall bracket, are the difference between hearing it and not.
Easy test that costs nothing: put your hand flat on the wall inside the room while the unit is running hard. If you can feel it, the path is the structure, not the air, and the fix is mechanical isolation.