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Split System Noise - Mitsubishi Three Head

I have a fairly new Mitsubishi split system with three interior heads, installed by a Diamond dealer with a good local reputation. We had some issues with the initial installation not heating very well and then having cooling issues in the summer which was finally tracked to a leak in the outside unit. They replaced it and the air conditioning worked fine. In cold weather now they can produce 100+ degree output.

This winter I have noticed more noise coming from the inside units. Essentially it only happens when it gets below freezing, happens more frequently when it gets colder and I believe it's also noisier. There is a loud whistle, like high pressure liquid moving through a pipe, which goes on for 10-20 seconds, followed by lower gurgling sounds. It can wake me up at night, and conversation has to stop.

We don't have too many cold days here, and I wanted the tech to hear it so between the weather and schedule, I finally got their senior tech out yesterday. Of course it was warmer, just about freezing, but before he left it did make some noise.

He says that the issue is the defrost cycle. It gets noisy as the system reverses flow to heat up the coil outside, which makes sense. Normal noises are understandable from machinery, but it seems to me that, especially when it's colder, it's unacceptably noisy.

Tough question, because my too noisy might be someone else's minor issue.

He's going to look into possible ways to quiet it down, but doesn't hold out much hope. Any suggestions?
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