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If this is an electrostatic filter with charged plates then it is usual to hear the 'snaps'. If it is doing ot continuously, then it is either failing or you have a hair or thread or some kind of foreign object that is shorting the plates.
If you have cleaned it and it is snapping continuously, the insulation may be breaking down.
Can you give me a make and model number and I can check for sure.?
That is old. I would suggest that it has started to fail. These units work by placing a large voltage (5000V+) potential accross a series of plates. Over time, the insulation will break down. If your is 20 years old and continuously snapping, I think it is bad.
Take one last look to make sure there is nothing shorting out the plates. If there isn't, you will need to get a new one.
Sorry. It's not the news you were probably wanting to hear.