Panels, guards, pipes, or mounting bolts have loosened. The compressor vibrates and the loose parts rattle, buzz, or clang.
What you'll see
Make sure the noise isn't from a failing bearing, belt, or coupling before dismissing it as 'just a rattle'. Listen carefully and isolate the source.
How to diagnose
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Touch test while running
With the compressor running (carefully, away from moving parts), lightly touch panels, guards, pipes, and brackets one at a time. When you touch the vibrating part, the noise will change or stop. That's your culprit.
Result: Noise changes when touching a component = that's the loose part.
How to fix it
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Tighten, secure, or add damping
Tighten mounting bolts, secure loose panels, add rubber damping pads where metal contacts metal. For piping resonance, add a support bracket or change the pipe run to alter the resonant frequency.
Don't ignore new noises. While loose panels are harmless, they can mask the sound of a real problem developing (bearing wear, coupling failure). Investigate any new noise.
Basic hand tools for tightening. Rubber damping pads. Pipe supports if needed.