I’ll be cruising along, say on the interstate, usually in very warm temps, when the noise from under the hood will increase by itself; it sounds like an increase in engine RPM, but there is no increase on the tachometer. After a few minutes, it may return to a “normal” sound.
Is it possible what you’re hearing is the electric (or clutch-driven) cooling fan coming on?
What you’re hearing is the fan clutch engaging when the temperature off the radiator reaches a certain point. Normal.