I had exactly the same issue with my N810. (I had forgotten about it also, since this happened long time ago).

The culprit was actually the battery moving within is place, not the back-plate itself.

The way I fixed it is: opening the battery back-plate (door), taking out the battery and putting UNDER the battery about 2 or 3 little "micro-balls" (for a lack of a better name) of that sticky gel like substance (mucuos like) that they use nowadays in magazines to hold sticked to the magazine inserts, coupons, brochures, etc... I just hope you know what I mean.

I decided on using this substance since: it is innert, you can clean it very easily, while is sticks stuff together it is NOT a glue (permanent) thing.

Let me know your comments.

Also, if anyonw knows the name of that gel substance, would be interesting to learn about it.

Kind regards.