"Groaning" noise from, I think, the transmission.

Hello,

I have a 2007 Toyota Tacoma 2-door, automatic transmission, and just recently passed 85,000 miles. I had my oil changed yesterday, and they topped off the power steering fluid. I don't believe they did anything with the transmission.

I noticed today that when I accelerate somewhat hard, as if passing someone, I get a groaning noise I hadn't heard before, almost as if the drivetrain is straining more than it should be. I can't really tell for sure where it's coming from, but it might be the transmission.

The truck seems to perform normally, acceleration is good, and the engine instruments are all reading fine - no overheating, no check engine light, etc. When possible, I tried gunning the engine while moving in neutral and the other gears (when at safe speeds) and didn't notice the sound. It only happens when I'm accelerating more than usual. Normal acceleration seems fine, and I didn't notice any unusual smells like burning, etc.

I checked under the hood to see if everything was okay, and I didn't notice anything out of place (I'm not much of an auto expert). I didn't see a cap for the transmission fluid (not even sure if it's under the hood), and the other caps seemed like they were on securely.

So does anyone have any ideas? I'm hoping to avoid an expensive repair as we're hopefully going to be buying our first house soon, and we're treading carefully with our finances to make it happen. Ouch! : )