I used my pops 04 2500 gmc to deliver a car yesterday. We covered about 700 miles with a load. Everything went grate and the truck pulled like crazy for a gas (6.0 ) truck.

At any rate. The trans or something at 81 mph + sounds like a auxiliary fan or something turns on. It does it in over drive and in drive. The Truck has 4:10 gears and stock size tire so in Drive it was about 4-4200 rpms and over drive was 3-3200 ( sorta hard to see the teach just right)

Anyway all the coolant gauges read cool, cooler than i thought to be honest. The motor never got over 200* says the gauge and the trans temp gauge only mover to the second little mark above 100, Which it had did that in the first 2 miles.

So anyway anyone got an idea of what i was hearing turn on and off.

 

you were hearing/feeling the A/C compressor

 

ah, So what does it just do it at those speeds?

 

no it does it at idle too. basically any time the ac is turned on

 

Got a 04 Quadra steer 3/4 ton yukon with the 6.0 and 4.10s, it's the cooling fan, it'll roar b/c the heat that it is feeling is causing it to spin along with the airspeed difference/flow thru the radiator.

 

Ah oh... that makes more sense... cause we didnt have any air on.. it was only 30* out side

 

Compressor will also kick on if you are running the defroster.