Did you already open it up? The D series is quite well to be opened (and re-assembled ;-)). Just around 15 screws…
It could be that somehow your processor got loose (try loosening/retightening the ZIF socket of the processor. This can be even done w/o removing the heat pipe. Might even be possible with only removing the keyboard.
If that does not help perhaps your processor is broken (can happen, but happens rarely). There’s quite some cheap replacement CPUs (~ 20 EUR) around. But this then means some more unscrewing.