The sensible thing to do if you've fitted an aftermarket filter and your engine starts to make a horrible rattle, is to put the stock one back on before you take it back to the dealer for the warranty assessment.

The manufacturer can only really refuse to honour the warranty if they can prove that the aftermarket part fitted has directly contributed to the problem. For example, if you fit a Kev Mod and your frame cracks, I don't think they would have grounds to refuse to fix the frame. They of course might try and wriggle out of it though!

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