My 2003 Cadillac Escalade ESV has 4 codes attached to it they are as follows: P2135, P1516, P0220, P0120 (these codes are from Auto Zone) I'm told either my trottle or trottle sensor may be going bad that was 1/7/13. Today, 1/10/13, I heard a distrubing humming sound coming from the rear of my truck near the gas tank area. Do you know what this could possibly be? Is this related to the trottle issue or something else that is going to cost me?