Analog audio · declared evidence
Which connection changes the noise?
Record safe, controlled signal-path A/B observations to choose the next hum or buzz test without assuming a ground loop.
Your controlled observations
One changed variable is useful evidence
A laptop battery comparison can implicate a power-related path, but it does not prove a ground loop. A replacement signal cable that changes the result implicates the original cable/contact path, not automatically the interface. If speaker noise persists with signal inputs removed, investigate that baseline before buying an inline isolator.
Do not defeat protective earth
Never remove an earth pin, use a mains earth-lift adapter, modify house wiring or open a power supply. Stop using equipment and seek qualified service for shocks, burning smells or suspected electrical faults. These instructions concern user-accessible signal connections only.
The planner does not listen to audio, generate test tones, diagnose electrical safety, or recommend a generic USB isolator. An isolator must suit the actual signal type, bandwidth, power and interface.