Noisy Daikin in Heating Mode.

Hello has anyone encountered a problem with a Daikin heat pump split a/c unit that appears to work happily in cooling mode, but when in Heat mode it's ineffective and the compressor sounds like a bag of spanners falling down the stairs.
Its not short of Gas nor is it the 4 way valve, Daikin Tech suggest a new compressor and exp. valve but this seems a bit excessive.

Any hints cheers.