Speaker makes a rather loud humming noise.

If you've replaced the LM4766 ICs and the humming sound returned, there could be a few possibilities to consider:

1. Grounding Issues: Humming sounds in audio systems can often be caused by grounding issues. Ensure that all components are properly grounded and that there are no loose or disconnected grounding connections.
2. Power Supply Problems: The rapid loud hum could be related to power supply issues. Make sure the power supply is stable and within the correct voltage range. Also, check for any issues with the power cables or connections.
3. Filter Capacitors: Check the filter capacitors in the power supply section and around the LM4766 ICs. Faulty or aging capacitors can lead to noise issues in audio systems.
4. Signal Interference: Electrical interference from other devices or cables can sometimes cause humming sounds. Make sure the audio cables are properly shielded and not running too close to power cables.
5. Speaker Connections: Double-check all speaker connections, ensuring they are secure and properly connected. Loose or damaged speaker wires can lead to noise issues.
6. Board-Level Problems: There could be other components on the circuit board that are causing the issue. Inspect the circuit board for any signs of damage, burn marks, or soldering issues.
7. Magnetic Interference: Make sure there are no strong magnetic fields or sources of interference near the system that could be causing the humming sound.
8. Professional Help: If you've exhausted your troubleshooting efforts and the issue persists, it might be best to seek the assistance of a professional audio technician who has experience with Bose systems. They can perform more advanced diagnostics and repairs.

Remember that working with electronics and audio equipment requires technical expertise and can involve risks, including electrical hazards. If you're not comfortable or experienced with electronic repairs, it's advisable to seek professional assistance to avoid potential damage to the equipment or harm to yourself.

Check the speaker cables for connectivity. Be sure to securely plug in the stereo jack/s at the back of your pc

Possibly a bad cable or bad ground or bad outlet?. Easy solution, try a different outlet and see if the hum still exists. Also, make sure your speaker cables are NOT run parallel along with, or close to any power cables, Speaker wires can and will pick up AC "Humm" noise.

Hello
Switch on your amp, keep its volue down to extreme low, and listen if there is still hum through the left channel or not. If still there is hum, the left side power amplifier section inside the amp;ifier is faulty. If htere is no hum at this state, hum enters thorugh the audio input terminal, and the input wires must be checked for open sheild connection. OK.

If I understand this correctly: with NO input or cable attached it hums; with an RCA cable attached it works. (What does 'some' mean in this regard?)

You say that connecting the RCA cable improves its sound. That tells me you're supplying a ground through the RCA cable for stray voltage in the sub.

It seems there may be a basic power issue if it hums with no input. Sometimes a given power source in a home may be wired differently than others or may insufficient grounding, which can result in hum. Try reversing the sub's power plug or move it somewhere else.

Since the sub contains its own electronics it is also susceptible to strong external magnetic fields. Keep it and any signal cables leading to it some distance away from other electrical devices like TV's.

Humming noises are often caused by a faulty Electrolytic Capacitor in the power suppy section of the subwoofer.

if the hum is only on the speaker s and not the headphones and you have tried the ground and ferrites I would suggest it is a amplifier problem. Either take it back for refund or plug a small set of amplified speakers into the headphone jack. i.e computer speakers or such- that will at least tell you if it is indeed the amplifier.

make sure you have the speaker connection right if you do then it is a ground problem try plugging it in a different outlet

check the cable 1st. If this speaker is powered, you could have a bad shield, or the power is causing a ground loop.
If the speaker is coming from an amp using speaker wire ie: 2 lead + - check the connection and make sure no fraid strands are touching each other on the rear connection.
In addition check your TV, Ap, And speaker system (if sub is powered independently) are all on the same curcuit.
a certain cause for ground looping resulting in hum.

First,
Disconnect any input cables from the amp. Turn down the volume controls and disconnect the speakers. Power back on. If you still blow fuses with nothing hooked up, have the amp fixed.
If the fuse holds, shut down and connect the speakers, leaving the volume controls down. Power up and see if your fuse holds. If you have hum, an output transistor is most likely shorted. If you have no noise, SLOWLY turn up the volume controls and listen for the hum. If all is quiet, turn down the volume controls and connect your input cables and turn the volume back up. Noise? check your cables!
Mister Phil