MHI Indoor coil Ice build up on defrost cycle

[NewmanRef]

We have been out to look at an MHI split system a few times with an intermittent fault. The customer is complaining of an indoor fan motor making a banging noise as though the blade is catching on something. Each time we have attended we are unable to manifest this noise.

Today my engineer has finally witnessed it happening... when the system is running a defrost cycle in heating mode, the indoor coil is freezing up and building up ice to a point where the fan blade is catching it. The ice build up is only big enough for the fan to catch for a few seconds, before the cycle terminates and the ice clears again.

Any ideas what could be causing this?

[install monkey]

****ted up indoor coil and insufficient gas charge, or lev issue-
surely theres over 25mm gap between outer edge of fan impellor and coil face