Metal sound when accelerating under 25 mph

[zfadinglifez]

This evening when leaving work I heard an odd noise come from my car, at first I thought it was a construction crew in the distance, but then realized it was coming from my engine. The sound was obviously metallic in origin and the best way i can describe tit is like dragging a metal pipe along the sidewalk, sort of a hollow metal clinking.
Not particularly loud, no noticeable difference in the way the car drives, but it is quite unnerving and concerns me.
The sound only occurs when I am accelerating at low speeds and while idling; once I take my foot off the gas it stops, and once I reach 25 mph or so I can no longer year it.
I filled up on the way home and added a container of dry-gas because the other day I noticed a slight shake on the highway that was slight enough to be explained by some moisture in the gas or an unbalanced tire. I am now wondering if that slight shake might be related to this noise, but the shake is very slight almost not even noticeable.
The car is a 2000 Ford Taurus with ~138k miles. I recently had a tune up done and serpentine belt replaced. The mechanic said he could hear a little noise but didn't think it was anything to be concerned about - not sure if this is related.
I have also been driving a lot more recently than before as I now work 40-60 minutes away and take the highway/run into traffic daily.
Any ideas on what might be causing this or what I can do to find out or fix it will be greatly appreciated.
Thank You

[shorod]

Does the sound frequency change with the engine speed or vehicle speed? If the car is in park with the engine running and you slowly increase the engine speed, does the noise appear over some engine speed range? Don't be alarmed if the engine starts to stumble when you get around 3,000 rpms in park, that is a safety feature to keep you from over-revving your engine with no load on it.

-Rod

[zfadinglifez]

I was actually able to see what was causing the noise, laying on the hot pavement is not fun but it worked. A flange had come loose so I drove to my local Midas and they cut it off free of charge. I have been told it does not need to be replace and I'm actually better off without it. Thank you for your help!