[mikew88gt]

I know when your thermactor line is off/loose it sounds like a nasty exhaust leak.

You say that you hear it more when you drive but not sitting at idle so try setting your parking brake and letting the clutch out a little in first gear to see if the torque from your engine is causing your exhaust to partially separate some where. If your engine mounts are loose or really worn out then maybe your exhaust is working where the headers and X-pipe mate. If there is any black carbon build-up around a joint it's leaking. Good luck.