2010 Chevrolet Silverado crew cab Z71 LTZ Vortec 5.3. Question: what causes the module above and to the left of the drivers head to hum at certain times. this module appears to be a speaker of some type. possibly for the Onstar?

Answer: It sounds what you are describing is the in cab temperature sensor. It does look a little like a speaker grill. This is used for the automatic HVAC air conditioning / heater system. The module is actually a small fan that samples the temp of the air in the truck to help the system determine and adjust the vent temperature output. I would say that the fan motor is noisy. This should still be under warranty it the truck has less than 36k miles.

1999 Chevy Silverado.
Engine size: 4.3 l
Question: how to replace the heater core.

Answer: For a step by step instructions, you're going to need a service manual. But in general terms, you have to remove the entire dash out of the truck to access the heater case. The heater case as well will have to be removed and then opened up to install a new heater core. You'll have to evacuate the air conditioning system since the evaporator core will come out along with the heater case. This is a very big job and isn't for someone without the right tools and experience. There's a lot of steps to take the dash out of your vehicle, including removing the trim, instrument cluster, radio, heater control head, any airbags, the steering wheel and column, all wiring harnesses, etc. Good luck!