This is the first method I tried when the rattling developed on my DT770s - IT WORKS! Have to admit, I was skeptical upon first reading, but I really do owe OP for this fix. Perhaps I was just lucky; still worth trying though. 

Newbie with a query ,   In a bit of a dilemma between two pairs of headphones: SRH840 and DT990 -- which do you recommend I buy?   I'm what you'd consider a 'basshead', so bass is my main concern. Any input will be much appreciated!