When the smart is driven gently and the transmission chooses 5th gear early to save gas, any acceleration then will cause a resonating on various components. An example would be a vibrating mirror.
If the transmission would be dropped to the next lowest gear, the engine rpm would rise a bit and the rattling may stop.
I have a little MAZDA B2000 truck and when I shift to a high a gear at a low speed, the little 4 banger with minimal torque causes the whole interior to vibrate so harshly that I must immediately drop down to the next gear. I wonder if this phenomenon is your problem, especially since the dealer is not concerned.