Condo Outdoor Unit Hums: Anti-Vibration Pads Flattened

Vibration felt through the floor of a high-floor flat whenever the outdoor unit runs, having built up steadily over several years. The unit itself sounds entirely normal from outside. Nothing has been altered on the installation.

Vibration that has grown gradually while the machine still sounds normal separates a transmission problem from a mechanical one. Every unit vibrates as it runs; the question is what stops that vibration reaching the building around it. Pads carry the whole of that job, and they do it by compressing and returning. A pad that has lost its thickness has lost its function entirely, and comparing one against a new pad is a two-minute check that needs no instruments.

A vibration pad works by deforming under load and returning to shape. Continuous compression, heat and age destroy that ability, and the material eventually takes a permanent set. Once flat, the pad conducts rather than absorbs, so the unit's ordinary running vibration passes into the slab and is felt as movement rather than heard as sound. The equipment has not changed at all; only its isolation has gone.

The pads are replaced, restoring the gap between the machine and the building it sits on. It is worth treating them as a scheduled item rather than something replaced when somebody finally complains, since they fail slowly and completely silently. On a high floor, where transmitted movement is felt far more readily, that schedule matters more than it does at ground level.