Check engine light is on and heard a buzzing noise coming from under the truck

You're the one standing by the truck and able to hear where the noise is coming from. We should be asking you these questions. If you can't locate the area the noise is coming from, start by having the diagnostic fault code(s) read. Those will give you an idea of which circuit or system needs further diagnosis. Many auto parts stores will read them for you for free.

That could be an automatic load-leveling pump, if your truck has that option. An emissions system leak detection device being run by the Engine Computer. You could be hearing the fuel pump. GM truck pumps are generally pretty noisy and easy to hear. That by itself is not cause for concern. The Check Engine light is telling you the computer detected a problem and it set a diagnostic fault code. You need to have the codes read to find out what the computer is trying to tell you.

That's a new one for me. In the past the fuel pump and fuel supply system was never monitored or had computer controls. That was one of the few things they hadn't yet hung an unnecessary, unreliable computer onto.

I don't have access to "U" codes and I can't tell you how that code would be diagnosed. Typically your mechanic would use a scanner to talk back and forth with the various computers and make them do things so he can test their operation. A non-communicating code such as this suggests the problem is with the computer controller and not the pump itself. You've already figured that out because the pump must be working if the engine is running fine.