GMC Troubleshooting Questions? Ask a Mechanic for Answers
Welcome, I'm Chris (aka Moose), my normal schedule online is around lunch and after dinner till early morning.
Does it seem like it is not running on all cylinders?
What liter engine?
Did you mean 5.3, 6.5 is not an option in the 07 GMC yukon SLT
I suspect this is going to be a valve lifter noise your experiencing. This engine will shut down cylinders under low power command and idle to reduce fuel usage. If the engine is not running optimum during this when in 4 cylinder mode it can actually run bad like what you're experiencing. I recommend using a stethoscope to listen to the valve covers to see if the source of the noise is coming from there.
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Thanks for using Just Answer, Chris