2007 Ford Mustang Engine tapping sound on start up in idle

I recently had my 1st oil change since my warranty expired and I had Mobile1 synthetic 5W20 put in. Shortly after I noticed when I start my car up in the morning that there is minor engine tapping noise when the RPM's lower to about 900-800 and stay there. The sound goes away aftger I start driving and when I start my car up again later on that day I don't hear it at all u ntill the next morning. Can it be something serious? I've taken really good care of my car and hardly ever red lined it.