for the past 2 wks, every morning when I start the tahoe cold, there is a rat - tat - tat - tat noise that continues until the engine warms up.

The engine seems to run fine, but I am worried that something is damaged or needs repair.

Is it carbon build up from bad gas?

 

It could be a couple of things, it could be the lifters pumping up with oil and taking a few seconds to loosen up, if thats the case running and oil additive may help with that.
It could also be the tranny more so the torque converter pumping up also.
How many miles to you have on it?
Do you regularly change your oil?
Has the tranny be serviced ?

 

do a search for piston slap