rattle under the hood

[Bevan]

That would almost certainly be the exhaust heat shielding. Start from the shield around the header and tighten every nut on that attaches it to the exhaust and follow down to the cat-converter.

[Juffa]

What Bevan said. If you have an early model (update your profile to tell us a bit more about you and your MX-5), also check the heat shielding near the exhaust manifold as the early models suffer from a broken weld which causes the shield to vibrate between 3000-4000 Rpm.

J

[Lucky_Luke]

I used to also have a rattle like you say but at exactly 2250rpm, i figured out this was the exhaust heat shielding as mentioned before... i just went in through the front wheel arch with a screw driver and pulled it back a bit and fixed it...

I know u mentioned u thought it was forward sounding but if you suspect it could possibly be you gearbox ...if its sounding bad enough, could it then maybe be your diff though?
In some diffs on the way out they can seem fine during acceleration but if theres excessive backlash or slop in a bearing they can make an almightly rattle when you downshift and engine brake or just back off the throttle... ... just a thought..