2005 Nissan Sentra begins to shake and rumble upon starting

[HomeChoppingNetwork]

Hi all, this is the first time I have asked for auto help on the internet; I hope I am following all necessary procedures.

Anyways, I have a 2005 Nissan Sentra with about 50000 miles on it. It runs just fine, but I notice that sometimes when I start it up, the whole car rumbles and shakes for somewhere between 3 - 7 seconds. The service engine soon light comes on periodically, but the owners manual says that making sure the gas cap is on tightly will take care of this, and so far that has been the case. Upon making sure the gas cap is tight, the light will shut off. However, even when the light is off the car will still shake upon starting it. Any suggestions?

Thanks in advance!