If you buy an Ecobee3 smart thermostat and, when you remove it from the box, it seems to rattle you might have a problem. Remove the accessory door at the top-rear of the unit. If does not rattle with that removed, you're in the clear. However, if it continues to rattle, return it to your vendor for replacement.
Yup, I'm speaking from experience. I couldn't find anything online if the rattle was normal or not, so I called Ecobee tech support and confirmed that something is Very Wrong.
I bought the Ecobee3 because it works with HomeKit, but also for its support of remote sensors. Those are essential to efficient home heating and cooling, in my opinion. I've written about another remote thermostat here, and if you want a less expensive model that works great, I recommend the Venstar Wireless Remote Thermostat without hesitation.
Fortunately Amazon is great about returns so I'll have a replacement Ecobee3 on hand soon. Stay tuned for more.