Knowing Daikin installations, I suspect a cheap bypass valve. Would need clearer pictures of the loft. Is it all. Radiators, do they all have TRVs

@clairelp I’ve received a couple of other answers from installers I shared this post with, and in addition to Ken’s thoughts above, here are some other angles and culprits to pursue:

• I bet pipework is clipped to timber in the attic this will reverberate through the structure.
• Is there a circ pump inside the ashp? That will most likely run constant if below approx 10degc ext air temp to frost protect. Some units require flexible hoses to prevent noise from emanating up the pipes. This may be required. Check system pressure too, there maybe what is called cavitation if the standing pressure is too low and a pump is in the loft. Try increasing the heating water pressure to 1.8 to 2bar to overcome it as the pipes riser high up the building and/or the pipes are too small.
• Or could be a faulty motorised valve, they can be noisy

Additional to all those good comments, especially the pipe clipping one, it might also be worth checking with the installer what the velocity in the nearby pipework is. A heat pump needs 3/4 times the water velocity compared to an oil boiler to deliver the same heat, and so if the pipe diameter is too small then high velocity water can end up pretty noisy which could be a contributory factor. You're aiming for around 1m/s, and no more than 1.5m, no less than 0.5. 

You may have a buffer tank in the attic, or the heat pump may be pumping directly to your heating and hot water system. If you do have a buffer then there will be another pump from that to your house. You could turn that off and make sure that there is a call for heat so that the heat pump runs, that would allow to you to see if the noise is from the heat pump side of things, or the bit of your system after the buffer.  

Trapped air can also be quite noisy - could also check if there's an auto air vent at the highest point in the attic, and that it's working. 

*(note that according to the OP this install does have initial flex hoses)  

@kenbone yes they all have trvs, they are all open. I will get some pictures.

@sune the pipes are 10mm from the manifold in the airing cupboard. 

yes there is a flexible section of pipe.

Thank you everyone for your help. Very much appreciated and some things for me to look into. We thinking If we can locate noise and it can’t be stopped we can try to sound proof walls ceiling or pump etc. 

will get some pictures in the loft and ask husband to look if pump near the expansion vessel, see  if this is the problem. 

@clairelp Hi, I have a Daikin 9kW heat pump and so might be able to help.

>> I would say it’s a humming droning sound, like a water pump circulating sort of noise.

I would say the heat pump and connected plumbing makes three kinds of sounds:

1. fan noise

2. compressor noise - like the humming of a fridge, but can be much louder

3. water flowing in pipes, radiators and valves

Which would you say it sounds most like?

1. I doubt it's the fan noise; that's pretty obvious and not very annoying, I think.

2. The compressor can be annoying, as it has a pretty bass-y chug-chug, particularly when the unit is working hard.

3. water noise from radiator values can also be annoying

Do you have a black MMI unit with two rotary knobs? If so, are you comfortable changing settings? 

There's a lot of settings in the installer menu some might help.

>> I just had to turn ashp off at the mains as it’s sound like constantly circulating water, even thought it’s all turned off via the app

When the outside temperature drops below about 5 or 6C, it will periodically pump water around the pipes to stop the water freezing. It shouldn't be doing it continuously (although there is an advanced option that will do that...), but may 5 minutes out of every 15.

Here are some pictures. I have lifted insulation of the top for pictures. 

The noise, we think is coming from the water circulating in the system. It can’t be the 10mm radiator pipes I don’t think, because you can hear the noise in the anti freeze cycle and I think that doesn’t use central heating but the hot water circuit?

The main problem is this anti freeze cycle as it’s goes all night and it’s extra loud and can hear it in every room. Loudest at bedroom. So this is when we have to switch power off to it. If the frost cycle isn’t going we can set back to 16 and get some sleep. If drops below 6 degrees we have to turn off completely. 

@squarepeg77 hi, thank you for your experience. Noise definitely not coming from ashp itself as I just went outside and can just hear it. Go in the bedroom and it’s noisy. Louder in then out. 

the settings are locked installer mode. I can’t change anything, The installers are coming Friday. So I can ask them. 

@clairelp there are three user profiles in the MMI: user, advanced and installer. 

You should be able to switch between them by going to User Profile and entering the appropriate codes. 

But if the installer is coming soon, you might want to wait for them. 

What I will say is that you should feel free to stop the heating cycle overnight. Unless it’s really very cold all day and night, your heat pump should be able to warm the house adequately when running for 12 hours a day, often much less. It might be a bit chilly in the house first thing in the morning. Probably better to be a bit cool, but have slept better.

A few more things:

our system does circulate water around the heating system when it’s cold. By default it can push the water at high flow rates and that can create noise.

There are options to lower the pump speed, which usually reduces the noise somewhat. 

Even if the TRVs on the radiators are fully open, the lock shield on the other side might not be. Balancing radiators often requires closing lock shields almost fully and then they can be noisy at high flow rates. 

Finally the auto air vent valve can be noisy as well. If you always have some radiators fully open, then I understand it is safe to close it. But might be better to discuss that with installers if you are not happy changing that. 

@clairelp

Don't forget that if you switch off the power to the heat pump the frost protection is no longer operable. Please ensure that the system contains anti-freeze or auto drain valves.