Could be anything. If its still under warranty just let them handle it. Mechanic will take it for a ride and see what he/she thinks it is and replace what needs replacing. You should be able to go with if you ask.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.