Westinghouse SP6390 Operation Manual page 10

FILTER INSTALLATION INDICATOR
6.
When either HEPA Filter or NCCO Reactor is improperly
installed, appliance will not work with making a "DiDi"
sound.
Solution: disconnect the appliance from the electricity
socket. Open the back panel to take out all filters and
re-install the filters.
After the installation is corrected, the sound would stop
and the appliance resumes operating.
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