1-3 gear humming??

I recently bought an 03 trd Tacoma, have had a 94 pickup and a 99 single cab taco, they were all 4 cyc. however my new one is a 6. I get a slight humming in 1-3 gear kind of similar to the sound reverse makes, not nearly as loud though. First gear it's most apparent, second a little less and third you can just barely hear it. My old man has not heard it but says it could just be the nature of the truck?? Have no noticed a change from the 1000 miles I have owned it, nor has anything seemed wrong.

My questions is, is it normal and or ok?? It has not gotten any worse and does not sound to awful bad but I have never heard it in the past on any of my 4 cylinders?? I have changed all the fluids except the front diff. so far, no change in the hissing/humming. I also have a k&n intake, I don't think it's coming from that though.

Thanks!

 

have you checked oil level in the trans

 

I have changed all the fluids except front diff. I am struggling to get the drain plug broke lose...

 

could just be noisy a friend of mine has a 2000 taco 2.7 4x4 and it sound like its going to blow up in all gears but has like 270k he said it started making noise around 150k

 

I would say it is more of a whining then a humming. Maybe it will go away when I get the front diff plug out and get the oil changed in there??