Calling all transmission experts

Just wondering if I should be concerned with the slight hiss that my 99 Prerunner makes when lightly accelerating. It's magnified when I put more of a load on the truck, suck as towing or backing a boat in at very low speeds. Is this just some slippage on the trans that I'm hearing and shouldn't worry about it, or could it be more? It's done it for the whole year I've had it

 

Vacuum leak?

 

Hiss doesn't sound like slippage... and IMHO, slippage is not something to "not worry about" unless it is within the range of typical torque convertor "buffering" when unlocked.
Slippage glazes the clutches, increases heat, burns fluid, and contaminates the fluid.
Just like a manual trans, slipping never gets better until the clutch is replaced and the pressure plate is resurfaced (or in the case of an auto, "plates" and "replaced."

But the trans is generally not going to slip when lightly accelerating. It'll generally slip under load.

 

I can faintly hear it when very lightly accelerating. Add some weight behind the truck, or try to slowly back a boat in while lightly accelerating, and it's louder. And that's exactly what the sound is like. Just a hiss, can't really explain it much better. Once I get up to speed it goes away, it's just the initial takeoff, or moving weight really slow

 

It could be anything ....not necessarily the transmission.

How are your tires & tire pressure?

How is your driveshaft? When was the last time you lubricated the zerks/ujoints?

Have you put it into 4WD? Does it still hiss.....??

 

Tires are new and pressured up. Just greased the driveshaft. Don't have 4wd but I'll take it if your offering . It doesn't make the sound when you rev in park or neutral, only when in gear