If the audio cables are connected to the disconnected passive speakers and you're hearing rattling sounds from them, it's possible that the speakers are picking up electromagnetic interference (EMI) or radio frequency interference (RFI). EMI and RFI can be caused by various electronic devices or even nearby radio transmitters.
When the audio cables are connected to the speakers, they act as antennas and can pick up these electromagnetic signals, which can then be converted into audible sounds. The rattling sounds you're hearing could be the result of this interference.
To troubleshoot this issue, you can try the following:
Check for nearby electronic devices: Move any electronic devices or appliances away from the speakers to see if the interference stops or diminishes.
Try different audio cables: Sometimes, poorly shielded or damaged audio cables can contribute to EMI or RFI. Try using different cables to see if the issue persists.
Ground the speakers: Ensure that the speakers are properly grounded. A solid grounding connection can help reduce interference.
Consider a power conditioner: Using a power conditioner or surge protector with EMI/RFI filtering capabilities can help reduce the interference.
If the problem persists or worsens, it would be advisable to consult with an audio professional or technician who can assess the situation in person and provide appropriate recommendations.