I have a 2004 Silverado Z71 4WD Crew cab with 180,000 miles on it. Recently I have been hearing a rattling noise every time I start it up in the morning when the truck is cold, once the truck warms up and I start it there is no noise, the noise sounds like it is coming from the back of the transmission, but I don’t think the noise is the transmission because there are not problems shifting or with slip. This is when the truck is still in park before I ever put it in gear, so about 5 seconds after I start it, it sounds like boots rattling in a can for about a second and then goes away. The truck runs and drives and shifts great, no noises while driving....thought it could be a bad catalytic converter, but I think the noise is too far back for it to be that. Any idea what it could be? Thanks in advance!

 

Check to see if the muffler heat shield has come loose. Both our 93 & 05 Suburban had that problem.