Weird noise happens randomly like the engine is working hard

I don’t really know what’s happening, I’ve read a ton on it and will probably just inspect all the things that it said it could be but figured I’d come to the forum to see if anyone else has experienced this. So yesterday I went trail riding and I went down a pretty decent size drop. It wasn’t rough on the truck...just kinda crawled down it. After, it sounded like something was working really hard and got loud for a sec and went away. It did it another time then didn’t do it again. Today I went to pull into a parking space and right as I was turning in it did it again. No check engine light, nothing really seems amiss. Maybe it was doing it before and I didn’t notice it? The only way I can explain the noise is... I can hear the engine running per usual and then it sounds louder for a sec like it’s running harder and then is normal again. I don’t even know if it’s the engine though.

 

Did the tachometer reading suddenly increase as well when this occurred?

 

I haven’t actually been able to see, it happens pretty quickly. I’ll try and see if I can catch next. But it doesn’t seem like it? Or rather it hasn’t caught my attention

 

How long have you had the truck ? Could be the cooling fan engaging on and off, my truck you can hear a difference and see about 200-300 rpm difference....

 

I have had it since April...guy I got it from was the only owner but I’ve had it for 2 months and some change.
Honestly don’t know why I didn’t think of the cooling fan...that seems pretty spot on as far as the noise goes

 

Air conditioner causing an idle up.