2000 Silverado 5.3L - Miss and Clatter

['00SonomaZQ8]

My sister's 2000 Silverado with the 5.3L seems to randomly miss, but it's not throwing a misfire code. Also, under any load the truck clatters like crazy. Her passengers have said the floor near the catalytic converter gets pretty warm too.

 

Where should I start with this?

 

I should add that the truck has had a full tune-up with GM parts including fuel filter, PCV valve, plugs and wires. I've even run premium and Lucas fuel system cleaner through it.

[00Silverado4x4]

I'd have those catalytic convertors checked for possibly being plugged up. If they are replace them and then run it again to see if it's still acting up.

[onecrazyfoo4u]

Easy way to check is just put your hand in front of the tailpipe to feel for good exhaust flow. If it's very low flow, then pull the cat off and check it with a flashlight. Had a catalytic convertor melt almost completely shut on my past car. You could hardly feel any exhaust coming out the tailpipe. And the car wouldn't rev over 2000 rpm.

['00SonomaZQ8]

I've felt for flow before and it seemed normal, but I guess I'm going to take it to a shop for her so they can run some diagnostics.