I had the same problem and did the following tests, with the back cover off.



The source of the heat is the SD card slot.



Test cases

(a) Reduced the number of files in my SD card. Heating still persists.

(b) Removed the SD card. Heating is minimal, slightly noticeable with the back cover off. Not likely to be noticeable with the back cover on.

(c) Changed the SD card to a different brand. Heating is slightly more obvious, but not as bad as the first test case. Not likely to be noticeable with the back cover on.



I am sticking to the second brand of SD card for the time being, so far so good but things may change when I put more files in. My first one was a Sandisk Ultra, which is well documented to have issues.