Okay, that sounds incredibly frustrating! You've done a lot of troubleshooting already. Let's break down what you've found and explore some potential solutions.

Understanding the Problem Magnetic Interference is the Likely Culprit

You're right; copper shielding in your guitar mainly addresses electrical interference (like radio frequencies). Magnetic interference is much harder to block. The fact that the buzz gets worse when you point the pickup at the floor, especially over the ground wire path, strongly suggests this. The fact that the electrical interference is isolated only to the recording room & the adjacent room, and that cutting main power to the house, except for one circuit, doesn't make it go away, definitely indicates this is the problem

The Ground Wire as an Antenna

Your observation that the ground wire is emitting EMF even with the power off (except for one outlet) is key. The ground wire, although meant for safety, can act as an antenna, picking up stray magnetic fields from outside the house and conducting noise back into your recording setup. The fact that some people have plumbing that causes this problem too, makes me believe that this is actually what is going on with the ground wire itself.

The 2.5 kHz Peak

The specific frequency peak at 2.5 kHz might give us a clue about the source of the interference. While it could be a harmonic of 60Hz mains hum, it's high enough that it could also originate from something else. Do you have any dimmer switches, fluorescent lights, or other electronic devices that might produce that frequency? While you've turned off breakers, are you absolutely sure everything is off? Even a wall wart plugged in with no device attached can sometimes leak noise. It also seems like the wire itself is acting as an antenna, for something.

Why is it isolated to the Recording Room (and the joining room)? Directional Sensitivity

Magnetic fields can be directional. The orientation of the wire, the specific wiring in your house, and even the construction of the room itself could be focusing the interference in that spot.

Room Acoustics and Resonance

While less likely for magnetic interference, it's worth considering that the room's dimensions might be amplifying the 2.5 kHz frequency, making the buzz more noticeable.

Ground Loops

It is possible that you have a ground loop that is isolated to that room. If the outlet in that room is wired in such a way that it has a different path to the ground than the other outlets in the house that can cause the ground wire to emit EMF, especially if you are running audio cables between devices plugged into different outlets.

Possible Solutions (In Order of Recommendation)1. Investigate External Interference Sources



Outside the House

Seriously consider what's outside the house in the direction the ground wire runs. Power lines, transformers, cell towers, or even nearby industrial equipment could be radiating interference. Although you have turned off all the breakers in the house but one, the interference could be getting inside the house from the outside.

Neighbors

Talk to your neighbors. Could they be running something that causes interference? They might not even be aware of it. I know this is a longshot.

Utility Company

If you suspect a problem with the power grid itself, contact your local utility company. They have equipment to diagnose power quality issues.2.

Ground Wire Treatment (Proceed with Caution - Safety First!)



NEVER Cut or Disconnect the Ground Wire

This is extremely dangerous and illegal. The ground wire is a critical safety feature!

Ferrite Beads/Chokes

Clamp-on ferrite beads (also called chokes) on the ground wire might help reduce the noise. These act as filters for high frequencies. Experiment with different locations along the wire. You can get these at electronics stores or online. Place them as close as possible to the location the wire goes into the floor or wall.

Ground Wire Shielding (Complex and Potentially Dangerous)

Only if you are absolutely certain you understand electrical grounding and safety, consider carefully shielding the exterior of the ground wire along the section that is causing the most interference.

How to do it safely

Enclose the ground wire in a grounded metal conduit or flexible metal sheathing. Crucially, this shielding must be properly grounded to the main electrical ground point (the ground buss in your breaker box) to be effective. It shouldn't create a separate ground path, which would cause a ground loop.

WARNING

Incorrect shielding can create a dangerous situation. If you're not 100% confident, consult a qualified electrician.3.

Improve Your Recording Setup's Grounding



Star Grounding

Ideally, all of your audio equipment should be grounded to a single, central point. This minimizes ground loops. Consider using a power conditioner with isolated outlets that create a "star ground" configuration.

Ground Lift Adapters (Use with Extreme Caution)

These adapters disconnect the ground connection. This can eliminate ground loops, but it also removes a crucial safety feature. Only use them as a temporary diagnostic tool to see if it reduces the buzz. Never leave them in place permanently. Use a multimeter to verify there's no voltage on the chassis of any equipment before touching it.

Isolation Transformers

Isolation transformers isolate the power going to your audio equipment, breaking ground loops.4.

Alternative Recording Locations



Accept Defeat (Temporarily)

If all else fails, can you realistically move your recording setup to a different room, even if it's not ideal? Sometimes the best solution is to work around the problem.5.

Noise Reduction Plugins



Software Solutions

As a last resort, invest in high-quality noise reduction plugins for your DAW. These can often remove significant amounts of hum and buzz, but they can also affect the overall sound quality. Izotope RX is a common choice.

Important Considerations



Safety First

Electricity is dangerous. If you're not comfortable working with electrical wiring, hire a qualified electrician. Don't take chances.

Consult an Expert

An experienced electrician or audio engineer specializing in studio grounding might be able to diagnose the problem more effectively.

Document Everything

Keep a detailed record of everything you try and the results you get. This will help you and any professionals you consult track down the source of the noise.

Be Patient

Solving noise problems can be a process of elimination. Don't get discouraged if the first few things you try don't work.

Specific Questions to Answer



What other equipment is plugged into the same circuit as your recording setup? Even seemingly unrelated devices can cause noise. Do you have any dimmer switches or fluorescent lights in the house? Try turning them off completely. What is the exact path of the ground wire in the basement? Is it near any other electrical wiring or metal objects? What type of wiring do you have in your house (e.g., knob and tube, Romex)? Older wiring can be more susceptible to interference. Is your house properly grounded according to local electrical codes? An electrician can check this.Good luck! It sounds like you're on the right track, and hopefully, with some systematic troubleshooting, you can eliminate that annoying buzz. Flag for review