This is the gasket located in the middle of your cyclone. It is accessed via removing the screws under the pre filter. Over time this seal can split, causing a loss in suction and for the pre filter to keep getting blocked.Dyson do not make this part£2.99
Dyson DC24 Vacuum Cleaner End Cap Assembly - Iron
If your cleaner head has started to vibrate more then the chances are the end cap has started to wear (you will see evidence of this).
This part is a great alternative to genuine. Will fit all versions of the DC24 vacuums