Noise problem on DAIKIN 4MXS68F2

[cool#9]

Hi everyone
I have a strange and boring problem on an installation consisting of:
1x 4MXS68F2V daikin outdoor unit
3x FTXS20G2 for 3 bedrooms
1x FTXS25G2 for the dinning room
We have an abnormal noise inside the copper pipes -just like a water flow- while running in heating mode.
The dinning room's piping are passing inside a bedroom through a thermoplastic profile (i don't what's the english for exactly, see the attachment and give the word in english please...).
We hear the noise when the bedroom's unit is thermostat OFF (fan OFF also) and the dinning room's unit thermostat ON.
Any idea well-appreciated
rgds
cool#9

[Brian_UK]

For 'profile' read 'trunking' or 'conduit'.

[back2space]

it is normal for all units to have gas circulating through the pipes when in heating even if some are thermostat off. The txv valves outside unit remain partially open on units that are off to facilitate oil return to the compressor. There is nothing you can do to stop this happening. If the unit is not workin properly then call an engineer otherwise try to maybe insulate the pipes better to provide noise reduction.

[brunstar]

In relation to this unit, have you chopped off the muffler on the liquid line? some people cut and braze instead of flaring and chop off the muffler, this will make the indoor units noisy, if this has been done you can order a new mufler and fit it, keeping in mind that some indoor units have 2 mufflers.

Also check the total length of pipework installed to see if you have enough refrigerant charge in the system.
If all of this is ok, it is the refrigerant passing through the indoor fan coil, but this is mainly on VRV systems that the EXV will stay partially open to give a minimum coil surface area as the condensing unit coil surface is sized much larger for cooling.