scenario- 2003 xterra 3.3 6cyl. auto trans. 137000 miles, cats replaced 1 yr. ago, along with rear main seal. Question- I have a rattling sound coming from underneath or engine compartment. Sounds just like the heat shields rattling. Happens at cold start for about 20 seconds or until I put it in gear and give it a little gas. Had the dealer look at the cats and said they were fine. Could it be something loose on the trans from when they did the rear main seal? I NEED HELP WITH THIS!!!