'96 Suburban 2500 4WD

I've got 2 questions about a '96 Suburban I bought in CA and brought to MN last year and all responses are appreciated.
My truck vibrates (bad) intermittently between 60 and 70. It'll run for 30
seconds vibrating like nuts and then it'll run smooth for 10 minutes. Speed changes seem to affect it. I've had it aligned, there's nothing loose on the front end that my mechanic or the guys a Tire America (2nd opinion) can find. Tires are balanced. Anyone seen anything like this?
Also when I start it in the cold it makes a rattling, hissing, clicking sound under the truck right in front of the cab. No one can find this one either. Is it possible that the CARB stuff is causing this in the cold weather?
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What is the engine and transmission?
Have ALL the bolts from the engine to transmission been checked, Including torque converter? What about the catalytic converter, is it original?

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