If we could see pictures it would probably help so we can better say yes this or no that.
It sounds like it could be a ground issue for sure. It also could be from a cold solder joint related to one of the grounds.
Cold solder joints are hard for us to discover but you have tools to check. First look for dull solder joints and re-flow them. Make sure there is no paint on the inside of the enclosure where the jacks connect and if there is just scratch it away best as you can. Use a DMM to check for continuity us the Diagram from our Crash Course Guide located in the guides section under Tips Tricks and Tutorials. It is a sticky at the top. Alternately there is a big green guide button on the home page or on the main forum page.
Let us know how you make out.