start up the car with your foot on the brake go from park to reverse to nuetral to drive then back to nuetral to reverse to park, do this several times and listen carefully if the noise goes away in park and nuetral, you probably have a torque converter bearing going bad. It sounds exactly like you describe, a water pump bearing can sound similar to this as well so as long as the water pump is not leaking and the temperature is not fluctuating its probably ok. A torque converter going out is common on gmc, in order to fix it the transmission must be removed, it is best to rebuild the transmission if it has a lot of miles on it. The sound will get louder and then get softer sometimes, but if you do a lot of freeway driving the converter is going to go out sooner or later.
