03 Pontiac Vibe rattling noise

I have a 2003 Pontiac Vibe, automatic transmission, front wheel drive, with 50,000 miles. For the past week or so I have noticed a loud rattling noise upon acceleration, in both drive and reverse. It only does it when I first start to accelerate. Once I get going it stops. Its really hard to tell where the noise is coming from most of the time it sounds like it is coming from the front of the car but every once in a while it sounds like its coming from the rear. Does anyone have any idea what could cause this? Thanks for any and all advise!!

It's the serp belt tensioner, very common problem. Fun is to replace, there is a five inch bolt, with three inches of clearance. Requires loosening the motor mounts, and raising the block enough to get it out and back in. Care not to damage the CV's though. Tricky but the only way to repair/replace.