Could have been an impedance mismatch due to driver not having enough power to reproduce bass clearly, but given it only happened on one side and never before changing the pads, it's not the case here I think. Another cause is, anything touching the driver while it maintains deep bass - which is hard to do - will likely cause trouble. Things like hair or pieces of whatever you used for the vacuum (btw, don't do this again ...). To check if that's the problem try to blow on it, to make the thing move. Listen again with the particular recording you've had problem with (which you didn't have before). If there's a hair inside then it might move and therefore makes distortion disappear, or to be more / less noticed.
I've read a thread on Head-Fi where a few people reported similar problems (link below). A whole lot of solutions offered therein, also a few pictures that might be useful if you feel like opening the cup.
http://www.head-fi.org/t/218922/dt990-rattling-calling-beyer-expertsDon't mess with the drivers unless you're sure of what you're doing. If you think you better not touch it any more, you can always send it back to Beyer.
Good luck !