First, did you mix transmission fluid with the engine oil? Also, is it a ticking or a knocking sound? That makes a big difference. The idea that you feel a rough idle could be a few things other than a major concern. An engine vacuum leak can cause a rough idle. Was the serpentine belt and the engine started to see if the sound was gone which would indicate if it is a belt driven accessory making noise?
How does the truck run once it is off idle? Has the fuel pressure been checked? Excessive fuel pressure can be causing issues as well.
Let me know these things as well as if the check engine light has come back on indicating the same or a different problem.
Until I hear from you, this link may be of interest. Take a look through it. It may help direct you are far as the noise you hear.
If it is possible, record the sound so I can hear it. Also, as far as taking the engine apart to check it is all going to be labor related and based on what is removed. For example, if a cylinder head is replaced, labor is showing 9.2 hours. The is a lot of money in time. However, if it is just removed, I can't imaging it would take more than 4 hours including inspection.
I attached a pic below that simply shows price for a new head and labor hours to replace one or both. Take a look at it for an idea on labor time.
Here is the thing. I realize that the warranty is 5 years/100,000 on the power train. Have you kept records of servicing? Have you used a Dexos approved oil? If you have, I would be contacting GM customer relations and request assistance even if it is partial help. The truck has very low mileage and this should never have happened. It's just a thought at this point.
Let me know what I can do to help. Also, if possible, let me know if the things I mentioned have been checked and if you can upload the sound so I can hear it.
Tuesday, May 5th, 2020 AT 8:27 PM