Is the noise a low hum? You said it sounds like something shorting out, so do you mean clicking? These things usually pick up AC induction, which would be a hum. If that's what it is turn off the breakers one at a time until it goes away. That will at least let you know what circuit is causing it. Also if it is AC induction shielded speaker cable grounded on one end should fix it, you're gaining nothing using CAT5. You could try grounding all the unused pairs in your CAT5, one end only and make them common to ground.
