GMC Troubleshooting Questions? Ask a Mechanic for Answers
Is it a deep knocking noise or more like a valvetrain tapping?
Is the check engine light on?
Any smoke while idling or on acceleration and if so, what color is the smoke?
Any fuel in the oil when you check the dipstick?
Any smoke on acceleration?
Is it a deep knock or more a tapping sound?
does it change in sound when accelerating?