DAIKIN R32 18000 btu/h refrigerant flow noise from internall unit during heating mode

HI, I have recently installed a daikin 18000 btu/h wall mounted air conditioner.Piping length is about 4,5 meters and the system was vacuumed before start.When I started the unit in heating mode there was a strange noise coming out of the internall unit like water flowing through the coil.There was no efficiency problem or other issue,but I would like to know if anyone has dealed with such a problem.
Thank you.

It is quite normal to hear the refrigerant as you describe

Thanks for replying,but this noise seems unusual.It is a continuous noise of refrigerant flowing and does not stop as long as the compressor is running.I did not test to see if the noise is present during cooling mode.I just wonder what is causing it and whether is a refrigerant charge issue. Thanks again a lot.