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?