Check the AC pipes along the fire wall, they will rattle and sound like the dash behind the stereo is falling out, mine were not even pushed into the white plastic clips, they were just sitting there flopping about.
The other noise under load almost made me sell my car, it was doing my head in.
It turned out being the steel pressure line from the clutch slave cylinder coming into contact with the brake booster, it does a Loop or 2 behind the booster to act as a dampner, it would touch the booster and resonate through the cab, it sounded like a bit of sheet metal flapping about under the dash.
The noise come into the cab through the brake pedal box and uses the brake pedal as sound generator, I found mine by drivng up long hills in 6th, loading it up, and pushing the brake pedal to the side, this made the noise go away, then Iwent hunting.
The line can be bent away, gentley, so it has 10-12mm clearance, you will never hear it again.
It was one of the greatest days of my life when I shut it up, the Dash rattles in this car were driving me insane, I had a HDJ79R cruiser with 300,000 km in a coal mine, with less dash rattles. I very nearly sold the car due to the rattles.
The only rattle I get now is if I have the A/C on 16deg on windscreen, after 10-15 minutes, the vents get too cold and start to rattle under engine load, if I turn of the windscreen and use face vents, the noise goes in about 5 minutes.