Engine Knock (lower left side)

[jjt_33]

A month after replacing the fuel pump, my wife's Expedition has developed a knock on the lower left side of the engine. I first heard it when I was idling at a drive-thru and the engine noise didn't sound right. When I got home I left it idling and got out to pinpoint a location. You hear it the most from the left side down near the tire, it's barely audible with the hood open standing in front of the vehicle and nonexistent from the right side. Any idea what this could be? Are there any specific components down there I should check (I can't find my Haynes manual for a diagram)? It's a 2007 with just over 81000 miles.