2000 S4
There is a metal on metal rattling sound whenever I hit a crack or bump in the road, over like 20MPH. It seems to be in the lower middle, not one side or the other. Ive been under it a few times and took the cover off, and didnt see or feel anything. It has gotten louder over time.

 

check your sway bar end links. the stock ones are plastic. it could also be a blown engine mount, control arms or cv joints.

 

Agreed , check the sway bar end links or the control arms. The upper ones get dangerous when they wear out.

 

Also check brake pads. My rear pads are so low that the caliper is loose and it makes a light/tin "tinkling" noise over every bump.

 

I also have a noise but mine is coming from the front and seems to appear when turning, stopping and taking off initially. I'm thinking steering stops need lubricating on the turning and hopefully loose brake pads from wear otherwise.

 

Is it a clunking? or a higher pitched 'tinkling'?