Noise after clutch replaced about 1000 miles ago---37,000 mi. on 2004 Nissan 350Z

I have a sound that is a rattle from under the car ever since the
Clutch was replaced. I have Stillen pipes from manifold back- non factory, and the sho said it was the baffles in the exhaust make this noise? The said they could od nothing about it.......
Problem is very noticeable when I idle in neutral

Asked by for the 2004 Nissan 350Z