When at idle or stopped for a light, sometimes there is a fast three or four knocking sound from rear of car...maybe trunk area.
What can cause that problem?
The EVAP emission system performs a series of tests upon cold start up to ensure that the EVAP system is operating normally. The EVAP canister vent solenoid valve will purge and cycle the solenoid causing a noise that might be heard inside the vehicle and perceived as a knocking noise
TSB #06-06-04-062 - (11/27/2006)
Models: 2006 Cadillac DTS
Subject: Knocking Noise That Repeats Three Times on Cold Start (Install New EVAP Canister Vent Solenoid Valve)
Call you dealer and ask if this repair is under warranty for your 07 DTS