Couple of thoughts:

1) Ground the shield on one end only (you probably already know that).
2) Bill asked about the type of noise: would you know a 60-cycle hum if you heard it? That would be a clue.
3) One last thought is to disconnect your cable and walk the run with a wand/probe to see if the noise follows the whole path or if it is isolated to one area.

"Press play and record at the same time" -- Tim Alberstein