Strange noise, only when hot weather

Recently I noticed a hum that lasts about 2-3 seconds, every 10 seconds or so. Noise loudness, duration, pitch, period, etc does not change with speed. Seems to occur only when outside temperature is hot, nothing in the cool mornings. Taco has 83K miles. Tires still good. Where to start looking?

 

are you using the air conditioning when it is hot?

 

Wouldn't make a lot of sense with the activity symptoms outlined above, however maybe an Electric fan or Air Pump?

 

How's the a/c working? The clutch could be short cycling due to low charge

 

A/C is working fine. I am not noticing any cold fluctuations. I run it pretty much constantly, even in winter. Was out running around today, temp 104, no noise. If I hear it again I'll have A/C checked. BTW, noise does not sound like its coming from within vehicle.

 

point about the a/c is it does not run continuously (normally). the clutch engages to operate the compressor when the interior temperature thermostat says it is time to cool again. all that stuff is under the hood...cycling on and off may be what you are hearing more often in hot weather and maybe not at all in cold weather.

 

Maybe buy a new serpentine belt for the fun of it.