For the past month or so I keep hearing like this odd rattling noise coming from like under the car as i accelerate the car. I also have some odd noise when the car is cold and I start it up for the first time in the morning, once it warms up it goes away but the rattling still is under the car. Its very annoying.

At first I was told by BMW around a few months back that my Catalytic Converter was going bad, I recently had it checked out by an indy, he says its 100% fine. I connected it to an ODB II and I get no codes for exhaust or engine (Thank G-d)

The rattling noise happens while in drive, park or neutral. no matter what I hear it. I checked out the Cats but they seem stiff and nothing seems to be rattling while its in drive I try and move them and their still in place, I hit em a little and nothing shakes, so it seems like there okay.....

What else can cause this? Can it be the O2 Sensors? :/

Thanks 4 the help!