Firstly you need a cleaning kit...C9000 these sell for around £7-8... then you can remove the aluminium reducer on the top of the filter,
Spray the cleaning fluid directly in to the filter and wash with warm water, let this dry, ( do not use hair dryer) and then re-oil with the second aerosol.
This is needed to be done around every 25k depending on use.
Best thing to do though is to make sure these are no leaves and things sucked up inside..if so just empty it....