Engine/Transmission (1994 - 1998) weird sounding 12 valve?

[vaicase]

i have a 94 with #5 plate in the stock place. i just came back from southwest kansas,1160 miles pulling a 12000 total weight trailer. i got 14 to 18mpg at 1750rpm. my problem is at between 1600 and 2100rpms steady,it sounds like the motor is cutting in and out.coming back empty,i ran 70to75mph it ran about 1800to2100 rpms. it sounds like the engine is cutting on and off. if i let up or punch it even a little it runs perfect. it has all the power i wantit got over20mpg on the way back no trailer. could it be something in the pump?in 3000 miles it only used 2 qts of oil. and let me tell you if i want to smoke it will fill up the whole freeway. by the way i have a stack,could that be it?because it runs perfect other than that noise. the tach stays steady. thanks

 

[Missouri Mule]

Is the engine making the noise or is it your fan engaging/disengaging?