JFNash

I've been noticing a slight noise coming out of the engine while the car is:
- in operating temperature
- transmission in D
- with a/c off

I'm no expert but I guess it is coming from the cylinders. Is it normal to hear the cylinders operating when the car is in the above conditions?

When I shift to N or P, or when I switch the a/c on, the noise disappears completely. This has been the case since I bought the car. Is it normal?

Another question: could replacing the differential oil be in any way dangerous (assuming that I'll be using the recommended oil grade & specs)?

A car tech told me that diff oil is only checked and shouldn't be replaced as if I change it, noises will start to come out before the diff becomes dysfunctional. Is it true?

Last thing, these days the avg outside temp is 90 and expected to avg 115 in a couple of months. I don't want the cooling system to get overloaded then the car overheats. The car still has the factory coolant.

Is there anything I can do to make the summer easier on the car?

One more thing: the car is '08 with 25K miles on it.

Thanks.