How to fix ground buzz

You have a ground problem. I suspect your cord. Replace it, being sure you use an instrument cable, and see it that doesn't help.

Most Amps will buzz or hum when there is a bad ground. It could be because you're plugged into an outlet with some other piece of equipment on it like a motor. if it buzzes on every outlet then there's probably a ground issue in the amp. You can try cleaning your 1/4 jacks with cleaner cause sometimes they'll get cruddy and make for a bad connection. I wouldn't recommend opening the amp and messing around with anything since you could zap yourself. If the amp buzzes withouth even having anything plugged in, then yeah, it's probably the amp with the bad ground. If it buzzes with instrument plugged in, then maybe it's a plug on your instrument since they have to deal with movement and can go faulty after a while. Good luck!

Looks like you need a new jack or you have a loose ground. That is why it works when you touch it - because you are grounding it. Check solder connections before you plug it in, and ungrounded instrument is potentially dangerous while using an amplifier

It is a definite pickup grounding issue causing hum/"buzz" which diminishes when touching strings. Check the impedance of cable jack inputs and cable which should be 1/4"mono input/output jacks) . A personal preference but effective is to NOT run guitar direct into mixer but through an external amp w/ a headphone out jack . Noise gating/preamp filter also will help eliminate the "buzz"/noise from pickups. It is unfortunately the nature of the "electro-magnetic" beast when it comes to electric guitar pickups .Hope this helps.

Check the ground wire

Or a bad solder connection on the ground wire on the jack, or shell/sheild on the guitar.

Here is the list you seek.
http://www.gibson.com/en-us/Support/TechSupport/Wiring Diagrams/

The voice coil on the cube speakers must be scraping against the magnet.Or the speaker cone must be damaged causing it to flutter at low frequency where there will be large cone movement. Try removing the phono jack from your musical instrument/ guitar and touch the tip of the jack with your finger. You will hear a hum, now turn down the volume and listen to the speakers very closely and see if you can identify the flutter. All the same you will have to fix the speaker or replace it. Have you ever dropped the speakers, if so the speaker frame could have warped causing the scraping noise.

The cause is a lack of proper earth grounding in the 8 track so your jack-lead is acting like a radio aerial. The solution is simple - buy a Direct Injection (DI) box to place between the guitar and recorder.

You have a grounding problem ! You need to BOND (electrically connect) all the metal parts including the cases and shafts of the volume controls and switches. OFTEN the cavity with the controls are painted with a conductive paint such as Aquadag or NicklePrint which is then bonded to the barrel part of the jack you plug into. Note, while this helps the buzzing due to touching the metaal parts, it will not eliminate hum if you get your guitar pickups into a magnetic field from transformers, etc.

Several things to try:
- Probably already have, but another guitar cord.
- Ground switch on the amp?
- I had an older guitar that buzzed due to the pickups. While touching the strings, all was well - I was acting as the ground then. The ground wire broke, so it buzzed no matter what. The ground wire ran to the metal bridge.
- Play louder than the buzz! :)