Mitsubishi City Multi - BC Box Noisy

I'm having issues with a noisy Mitsubishi City Multi 407c - 10 port BC Box. At first I thought t was a chattering solenoid. But I have tried disconnecting all the solenoids one at a time but it makes no difference.

The noise comes and goes and sometime lasts for about 20 minutes, it appears to happen when one of the units shuts off - but I have tried running the system with each unit off in turn but the noise still occurs. Noise is a loud hum / vibration and almost sounds like gas being forced through a partly open valve.

I'm a FM not an AC engineer, our service company -want to swap out the BC box, but at almost £4k - i would rather try to find /fix the cause in case it is something minor.
BC box is in an open plan office area - so the noise is a real issue.

Gas charge is spot on - any ideas?

If you are not an A/C engineer....how do you know the gas charge is spot on?

I did my F gas last year - to allow me to check gas charge and leak test systems. I can only go my commissioning data for initial charge calculations, but as the system has run fine for around 15 years -I'm presuming its accurate..

The only 100% way to be able to assure Mitsubishi that the gas charge is correct, is to reclaim it all and weigh it, then put it back in.
It might be a software issue, but as the system is 15 years old, that would seem unlikely.
If you are unable to satisfactorily pinpoint the cause of the noise, you might have to engage Mitsubishi site engineering to attend and put forward their opinion after running the monitoring tool.
At least that way, if you have to replace the BC box, you are working on an informed decision rather than guesswork.

The correct way to check the gas charge is to fully recover the system and weigh the amount recovered.

This weight can then be compared to the commissioning charge amount to determine if the system has lost any gas.

On large VRV / VRF systems, this is the only way to check that the charge is correct and it is one of the things to check when you have issues such as the one you describe.

thanks

I recovered the system on Friday - and the charge was spot on. As you suggest - I will contact my local Kooltech engineer - to get him to connect the Mitz monitoring tool, and see what that show up.

Not heard back from Kooltech yet - but phone mitz spares - and they advise 12 - 14 week lead time on a replacement box :( - maybe I better order 2 !!