I have a 2003 pontiac vibe, and I maintain it very well. however lately when I first start it up, more so when it is cold out side, it has a rapping sound coming from the front of the engin. as the engin gets warm it quits. I check underneath the engin and it is not a rod knocking. I am pretty good a fixing stuff but I doint like ripping in to somthing without knowing what causing the problem. I think it might be the timing chain tensinor.