2003 Mitsubishi 2003 Eclipse cold start pinging/rattle noise

Engine problem
2003 Mitsubishi V6 Front Wheel Drive Manual 28000 miles

I have a 2003 Mitsubishi Eclipse GTS with about 28,000 miles on it, 5 speed manual transmission.
The car is completly stock.
Just recently, the car has been making a pinging or rattling noise when I drive the car first thing in the morning/cold start.
I can't tell if it is coming from the engine or the transmission, although it sounds more like the passenger side of the engine, but close enough to the bulkhead that I can hear it clearly inside the car with the windows rolled up.
It makes the noise the entire drive to work (about 5 miles - 15 minutes of driving), however when I leave work to go home in the afternoon, it does not make the noise.
It seems to be related to cold starts, but it does not seem to go away after the engine has warmed up, at least in the 15 minutes that it takes me to get to work.
However, as I mentioned, it does not make the noise when I drive home, but the daytime temperatures are much warmer then the morning temperatures.
About 2 months ago, I had the serpentine (sp?) Belt replaced, so I thought I should mention that.
Does anyone have any ideas as to what this might be, so that I may have some inside info before I take it to a mechanic?
Thank you in advance for your assistance.