Nothing, they have done what they can but it’s still NOT FIT FOR PURPOSE!. 1/ it’s noisy 2/ your cold. As said above your pissed off with it, quite rightly. I would get on to however paid fir it, gov scheme or whatever and tell them it’s NOT FIT FOR PURPOSE despite many visits by engineers. Hopefully it will stop other “customers” getting the same crap. It’s not as if you have not tried.
Unfortunately it’s circumstances like yours that makes people sceptical of ASHP,s. As was said many pages ago, they are only wrong if designed or installed incorrectly, there are plenty of people like me that are very pleased with theirs.