If the hydronic piping to radiators or UFH is correctly sized, there should be little noise. Noise tends to be caused by one or more of these factors:

1. Air in the system. Air circulating round the system will generate noise. Air is common after commissioning, draining down and refill, but should be bleed out automatically or manually in a few weeks. Once the air is out and system is at correct pressure, it should be practically silent.

2. Undersized pipework with high flow velocities. Can be common in retrofit e.g.: fired boiler to heat pump, where existing house piping is re-used that was sized for lower flows and it's not practical or too expensive to rip it all out and go up in size.

3. TRVs or automatic bypass valves. These generate noise when they are slightly open and passing flow & pressure. TRVs can and arguably should be avoided, and automatic bypass valves should also never operate, even if fitted, on open loop systems.

4. Poorly installed or mounted secondary circulating pumps. Correctly installed, they are remarkably quiet, but a bad install can create a noisy pump. Avoid mounting a circulating pump onto a stud wall, suspended floor, or loft truss frame. These structures act as sound boards. Fit to blockwork where possible. Use insulated rubber mounting pipe clamps on any pipework either side of an in-line pump not mounted on a block or brick material support. These simple and cheap little things often get overlooked when installers are up against the clock in your home. They won't be in bed with you the first night it's operating and that thoughtlessly positioned secondary pump is drumming away. (Well, they might be, but that's your business....!)

You should expect nothing more than a barely perceptible 'white noise' of flow circulation when operating. For low flow temperature systems operating for long periods, creaks and bangs due to pipework and radiator thermal expansion and contraction should not be expected either.

@allyfish Thank you for your comments.

I have only just started to use the heating side of my installation but fear that I have exactly the situation described namely a quite loud humming from the system when heating.

To mitigate whilst I discuss with the installer, I have a set back for overnight otherwise it would keep us awake.

I have a 25L buffer and secondary circulator pump in the loft. There's not much noise in the loft.

Mitsubishi were happy with my buffer as "I have microbore to the radiators".

The noise appears to travel down the pipes into what was the airing cupboard.

Hopefully not, but I suspect it's caused by your reason 2.

TRV's are all open

Bit confused about your comment concerning mounting circulator pumps as mine is simply suspended by the pipework.

Posted by: @richard24738

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Bit confused about your comment concerning mounting circulator pumps as mine is simply suspended by the pipework

That's normal for smaller pumps, such as home central heating ones, the pipework takes the weight, but the pipe supports should then have some cushioning to limit noise & vibration transmission into whatever they are fixed to, especially if it is not a solid block or brick wall.

@allyfish Thanks. One end of the pipework has a bracket to a roof strut and has rubber at the joint to the joist.

However the other end (out from the pump) goes for about a foot and then an angle down I think to the airing cupboard. At the angle, there is lots of insulation but no fixing and the insulation rests against the joist.

I am going to try to attach a photo.

@richard24738 insulation won't transmit noise. But any support between piping and roof space would benefit from being vibration absorbing rubber lined pipe clamps. It's assumed the pump is adequately supported by the piping, pump shaft horizontal, etc. Microbore, or smaller bore 10mm pipework will run at quite high velocities with ASHPs in order to get sufficient flow to convey the heat energy to the emitters with a low flow and return temperature loss. Not much can be done about that. Sometimes a secondary pump and hydraulic separation is needed to allow a slightly different flow rates and temperature drop delta T through the ASHP and the radiators.

@allyfish Thanks. I was hoping that having no clamp was the reason for the excessive noise.

Guess I'll need to wait for the installer to visit.

Not sure what the last part of your comments means.

I have two circulator pumps; a Grundfos primary and a Wilo secondary.

The secondary is installed after the buffer for the heating circuit.

My system makes a constant hum. Can't hear anything outside the house heat pump is mega quiet.

Buffer mounted on a main joist and to a board on the back and that board is bolted to a upright joist in the loft with a pump floating on pipework next to it making noise you can hear all through the ceilings. 

I'm 99% sure my external pipework isn't fully sleeved and insulated through the wall so the last 20cm are just in mortar transfering heat and noise. 

The airing cupboard seems to collect all this noise and amplify it so probably have to sound proof this at some point. 

I can live with it for now but definitely need to save up some cash 🤮 and get someone to sort it as the installer doesn't think there is anything wrong with it (basically it will cost them money to sort I think and they don't want to do it)

@burtis Hi. In my case I don't think the noise is to do with the external pipework as we would then get the noise when just running DHW. Just a small hum when DHW is running.

Also, there is virtually no noise in the loft where the primary and secondary pumps are located when having DHW and/or Heating

Yes, the noise is definately loud in the airing cupboard.

As you say, we can possibly live with it now mainly because the autumn temperatures mean that much of the time the heat pump will be in an idle state. However I dread when the weather gets colder and the heat pump is in theory on low and slow throughout the day.

If my installer can't/wont sort it out then I will have to find someone who can.

Posted by: @burtis

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I'm 99% sure my external pipework isn't fully sleeved and insulated

Is there flexible hoses between heat pump and the solid pipe?

@johnmo in my case yes there are I made sure, they originally broke one and hard piped it but made them get a replacement Flexi.

Regarding the noise in my airing cupboard, my installer will visit but in the meantime he suggested reducing the Wilo Secondary pump speed slightly. It was on Medium House which Wilo say is for 20 radiators.

I have 16 radiators so slightly reduced the Wilo speed (towards the Small House for 15 radiators) and eureka the noise had gone! No more waking up in night. All rads appear to be heating as normal. No short cycling.

Of course the change has to be validated by the installer and explain why it was on the Medium House setting in the first place.

I thought I would mention this just in case it can help someone else.

@richard24738 Good to hear the noise has gone - must be a relief to you. Is this pump the Wilo Pico; that would fit the description? I have a Pico and my installers set it (initially on the higher end of the range) and I found that the Delta T was only 1.5 - 2 degrees C so after I had balanced the radiators (well approximately at that stage) I started to slow the Wilo pump in small increments until I attained 4.5 - 5 degrees C Delta T. After attaching some digital thermometers to the four inlets/outlets of the LLH, I was able to refine the setting and achieved approximately 0.3 degrees difference between heat pump out to radiator feed from LLH and nearly as narrow a difference on the radiator return and the heat pump return. There is a topic on this forum in which I relate this and I will try to find it later then post the link. I might add that our system is very low noise and unless I open the airing cupboard door and look at the Wilo Pico display, I am never sure if the pump is running or not. Regards, Toodles.