At the end of 2024 my family started a project to remove natural gas / methane service from our house. As part that we began to replace our gas furnace with a heat pump.

The contractor mounted the outdoor unit on the wall of our house in a side alley that kept it out of the way.

This helps save space on the ground and mounting a couple feet above ground gives space for the condensate to drain when in the unit is heating our house. This does not help when the unit is a source of vibration and with the mounting causes structural vibrations in my house.

Mitsubishi has a helpful application note regarding the frequencies of the compressor at various times. During startup of a cycle in the 20-50 Hz range my house would vibrate and could be felt in various rooms through the house.

Pads (the blue/black pieces) were placed in the mounting to help address the issue.

They... might have helped. It's hard to say, I don't really have the tools to measure structural vibration. They did not help enough to avoid driving me crazy every time the compressor started.

What did solve the problem was moving the mounting to the ground.

This addressed the structural vibration issues to the point that I could no longer feel anything in my house.