Rattle Noise at 2k RPM - help!

Hi all,

I bought a 2010 Mazda 5 a couple months ago with 108K km on the clock.

Had to get the sway bar bushings replaced and the passenger side engine mount. These helped solve some knocking and vibration issues. However, there remains a really irritating sound that occurs in the 1900-2300 RPM zone when pressing the gas. Engine sounds great otherwise, at lower and higher revs. Idling in neutral, the engine is practically silent. I cannot replicate the sound in neutral regardless of revs.

The sound appears to come from under the central part of the dash but hard to say exactly. It is a light clickety, rattling sound. Imagine shaking grit in a small glass jar. I have taken two mechanics for a ride and they hear it but say they don't know what it is - annoyingly it is always quiet when I show somebody else! Nothing major they say, like the engine. It is costing me money to be told they don't kkow what it is. It's getting on my nerves and I want it solved.

I assume such a sound, at a given rev range, is related to vibration of a loose part, like a screw maybe, or two materials connecting and resonating. I've tried exploring under the hood to find loose components. I braced the hood prop and radiator with tissue as these seem a little loose, but the noise still happened. I've been holding all different things in the cabin to see if I can isolate it. No luck. I did wonder if there is something in the trench beneath the wind screen, connecting under dash to under the hood (by removing the windscreen cowl)...

Any ideas? I've searched extensively online. Some people mention similar but the threads never seem to reach a conclusion. The car is otherwise fine but I hate it at this point and want it sold - except I'd sooner fix this than try to stiff somebody.