Hello

I'm trying to diagnose a transmission issue. No lights/codes are thrown (basic scan tools).

• EX 4cyl automatic sedan
• No (known) transmission fluid changes
• No issues with getting into gear
• Accelerates very well when shifted manually (1 -> 2 -> 3) from 0-60 in about ~8 seconds.
• Makes considerable noise when accelerating from a stop in Overdrive/Automatic - if throttle is 50% or more, the car sounds like the engine is violently vibrating.
• Almost zero acceleration in higher gears, the transmission doesn't seem to downshift in response to the throttle. As an example: 5th gear, 2k RPM, 50 MPH --> floor it, excessive noise (similar to vibrating) comes from the engine bay and the car hardly increases speed. RPM goes up, that's about it.

I'm somewhat at a loss. The lack of acceleration seems dangerous and is hard for me to believe it's by design. I've had the car for a few years, this issue seems to be somewhat recent. Though I have been driving faster cars this past year due to considerable traveling. Maybe I didn't realize what I was missing??

1. Is this normal?
2. Failing torque converter?
3. Fluid change?
4. Solenoid or sensor?
5. 

Thanks all!