Surging?

So I was driving along today on a flat, smooth asphalt road having the normal tire hum from Duratracs and I'm just holding steady at 25-27 mph in a residential area and I don't know if it is the engine or the trans but it seemed to be louder and almost surging (but not surging in the sense of accelerating faster) in terms of noise and effort on the part of the engine or transmission. I've tried several times to repeat it today at similar speeds and conditions but I can't seem to reproduce it. Did I just happen to find the magic speed where the trans wasn't quite sure whether or not to shift up a gear or is it something with fuel not getting to the engine properly?

Or am I just full of shit and making something out of nothing?

 

Was it pretty hot outside? Maybe the fan clutch engaged briefly...

Spruce Mica!!!

 

I think it was only 60 degrees today so fairly mild.

 

If you couldn't repeat it, I would put my money on the fan anyway.
Unless you were coasting down hill slightly and the injectors turned off for a moment? That makes them quieter momentarily...

 

The needle bearing hums between about 30 and 45 mph if you have that issue.

 

I'll keep an eye on it and see if it happens again. It was just one of those things when you are super familiar with your truck and you're like "That's a new noise or that gear change didn't feel typical".