I've just serviced a DC19, it's about 3 years old but has never had the filter washed (the owner didn't know there was a filter, let alone where it was-she 'thought these modern Dysons were self-cleaning'...) it was running, but the motor sounded like it was being strangled.
I've stripped it right down and washed most parts (cyclone dismantled, motor bucket opened) to remove all the debris and dust that had built up inside. New HEPA filter, and the pre-motor filter washed clean. Upon reassembly, the motor sounds fine and suction is restored, but there is a horrible howling noise, which I assume is a seal somewhere leaking air? Removing the cyclone stops it, but I don't think it can be the cyclone itself, as that only has the one seal, which was fine. I'm thinking maybe a seal has gone dry/shrunk a bit, and it was only the dirt that was sealing it tight before?
Can anyone suggest where I should start searching please? Is this a common problem on the DC19? I've only ever encountered it on the DC07 before.