2008 Ford taurus x thumping/knocking noise

[veecorr]

I recently purchased the 2008 Ford taurus X . After driving the car for about a week I noticed that once the engine warms up, about 30 min into my drive. I hear a loud thumping/knocking noise coming from the engine. I have noticed that the noise is more prevelant when I am stopped (idiling) with my foot on the brake or when I am in park. If the car is in drive but I am stopped and take my foot off the brake the noise is somewhat more faint.
I took the car into Ford and they could not diagnose what the problem is (or if there is a problem). What the did tell me was the noise was not coming from the engine but appeared to be located where the exhaust and engine meet??
Has anyone heard the same noise?? Any suggestions????::chris.correia@sympatico.ca