Common problem due to a plastic gear starting to wear under the seat. Several threads have been started about this - it is possible to replace only the worn gear, but the dealership won't do it - they will replace the whole slider assembly. If you can get the dealership to do the work on warranty, great. But otherwise they quoted me about $800 to remove and replace the entire slider assembly under the seat.
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There are a couple more threads I found but not a whole lot of good info in them.