Sound from front catalytic converter

I have a check engine light on many of you know the 0420 catalyst system code. My 06 is a 4.0L and has 160,000 miles on it. When I start it after it has set awhile it sounds like the passenger side cat is humming and rattles when I take of in drive. Once the truck is at operating temp it quits. Anyone experience this? The local muffler shop said they would not weld on new ones for the fact that it's a pain in the butt unless I had deep pockets. At a loss