High-Rise Unit Hums: Internal Compressor Mounts Perished

A deep vibration from a high-floor outdoor unit that has worsened over years, with the machine still cooling entirely correctly. The feet under the casing are in good condition. The noise appears to come from inside rather than from the mounting.

Sound coming from inside the casing while the external mounting is sound points at the second layer of isolation. A compressor is not bolted rigidly to its base; it sits on small rubber mounts of its own. Where those have perished, the compressor's ordinary running movement reaches the casing directly and the whole shell becomes a sounding board. Feeling the casing while it runs distinguishes that from anything under the unit.

Rubber ages under load, heat and movement, and a building that sways at height keeps its equipment moving even when nothing is running. The small mounts under a compressor take that continuously, and they harden and crack years before the larger feet outside do. Once they have, the compressor is effectively bolted to the casing, and every bit of its running movement is broadcast by the panels.

The internal compressor mounts are replaced, which restores the isolation without touching the compressor itself. Naming which layer has failed matters here, because the external feet are what people look at and they were never the problem. A unit condemned on noise alone, with sound feet underneath it, deserves to have the casing opened before any quote is written.