If you don't hear it at idle and only under load, it almost sounds like an exhaust leak. Check your flex pipe and your cross over pipe for damage or leaks.
Also, get a mechanics stethoscope and have a buddy get behind your wheel while it running, apply the brake, put it in gear and load the motor. Then try to pin point the noise. Make sure its a buddy that doesn't have a life insurance policy on your or an axe to grind...that foot coiuld slip off the brake real easy!