Ok, this just started today. We where driving our AT around just enjoying the weather. When I pressed the accelerator to pass a car going to way to slow, the car down-shifted and a loud rumbling sound started and the car didn’t really have any pickup. The sound was kind of like tumbling rocks. I let off the accelerator and the sound stopped.

We pulled over the put the car in park. Revved the engine up slowly and then quickly and the sound was not heard. We started driving again and the same thing happened when we accelerated hard.

I could not tell if the sound was from the transmission or the engine compartment. I looked for something loose and dragging, nothing found. This sound is not heard while driving under normal conditions at any speed, only when we accelerate hard.

Any ideas what the problem maybe? I guess we will take it to the dealership on Monday.

 

https://www.rx8club.com/rx-8-discussion-3/engine-rattle-when-revving-over-5k-85866/page4/

https://www.rx8club.com/series-i-tech-garage-22/buzzing-sound-over-7k-rpms-87221/

Check those two out. In summary, I have that problem too and nobody knows what it is. I still have to take mine to the dealership again, they said I could be using bad gas, but I've tried other gas stations and the sound is still there.

 

Sounds like your car is pinging under hard acceleration in 2nd gear at top RPM. Try to use a full tank of 93 octane. Probably you are using 87 now... is good for better milage and performance - UNLESS your car pings on it. If it does, you need to compromise and go with 89 (I did) or even 91-93.
If that does not work, your cooling might be shot - change the coolant.