Noises from indoor parts of ASHP keeps waking me up

So, we've had the heat pump since January. It's a samsung air source unit. The outside bit is nice and quiet, I can't hear it from the house. Unfortunately the sound from the pipes / water tank / pump I don't know which is enough when it's running to prevent me from sleeping.

I set it to the lowest knockback temperature I can, and it doesn't come on at night, then we're all good... Until, it starts coming on in the middle of the night for seemingly no reason. Even with the heating and hot water switched off it still turns on for about ten mins at a time, regularly. About 5 times last night between 3am and 7am.

I wondered if this could be the defrost running, as it was pretty cold last night (I'm in the UK) but would that make noise inside the house?

Another thing, I don't know if it's related, but it certainly contributes to the waking up is the weird noise it makes when it starts up. It sounds like a toilet repeatedly flushing for about ten minutes.

In the mysterious night startups it just does one or two of the "flushes" which wakes me up, and then has the hum of it running for about ten minutes then stops. The video shows you what it sounds like.

So... What the hell is going on? And how can I make it shut up at night so I can sleep? Please help me, someone.