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There has been several good threads on this subject matter.

Carrier Greenspeed....

IMO

I read some posts on this forum (using the search method) I also read another forum.
Someone complains about the noise, someone says to overdimension the heat pump, someother that the problem was the muffler not correctly installed.
I was looking for some owner which would be able to give me is impression.

There are some posts on YouTube you should watch. Search for Carrier Greenspeed noise. I heard one that was very noisy, but two others that were quiet.

I believe there is a muffler the contractor can install. You have a better chance of having it installed correctly by a Carrier Factory Authorized dealer.

It is a bad idea to install an oversized heat pump so that it operates and lower capacity in order to reduce the noise.

You are overly concerned about the noise issue. Yes, it has been a problem on a very small percentage of installs. I believe it has been corrected by Carrier. Discuss with your dealer.

IMO

I spoke with two different dealers, and they told me that when the temperature drops below -15C the heatpump make a noise since its working harder.
One of them told me that he decreased the RPM and the owner asked to put back after receving his next electricity bill.
They also told me that probabily that the difference between summer and winter make more relevant noise. During the summer the heat pump is too silent and someone thinks that it does not work.