"Chugging Noise from Engine" - 04' Av Z71

2004 Chevy Avalanche z71, 5.3 vortec. In need of some help diagnosing a noise coming from the engine. To best describe it: it sounds like an old Steam Engine Chugging. The sound matches the speed of the idle or when at speed down the road. Quite loud. It's different from the Vortec "tick" that we know and love. It is loudest from under the engine or from in front of it with the hood open. No rough idle, no shaking or rattle just a loud noise. The belt looks ok. I have had a code for the catalytic converter for some time now. The truck has close to 250,000 miles on it. Thanks for any help on diagnosis!!!

I'll go out on a limb here and suggest you may have a clogged cat.

enoniam said:

I'll go out on a limb here and suggest you may have a clogged cat.

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agree... as exhaust is restricted at cats

Change converters and o2 sensors.

Also check for exhaust manifold leaks.