I see a lot of split systems with outdoor unit on brackets.
Usually, there is a piece of rubber (looks like chunk of
conveyer belt) between the feet and the bracket. But the bolts
are metal-to-metal on the bracket and transmit vibration.
The system at my house was done thus. So I put some rubber washers
under the bolts and nuts, to insulate it from the bracket and
outdoor unit, and also ensured that the bolts were centered in the
holes and not touching the sides. Now I hear less vibration inside
the house. Why can't the tradesmen do this?
I have only seen one other installation that used rubber washers.
I also found one that had airbag isolators about 6 cm thick;
but that was on a tycoon's mansion.