My 98 E320 makes a sound (popping sound as if gas is released from under the hood)when I start the car. Engine sounds little louder than normal. My wife also noticed yesterday that when she will stop at a stop light engine will rav up. Any ideas what might be wrong? Does it sound like EGR valve? If it is can somebody please tell me steps to replace it.

Thanks in advance.
Newbee2000.