DIRTY MAF 

Clean your Mass Airflow Sensor with some MAF sensor cleaner or contact cleaner

clogged secondary shutter valve [ssv]

The secondary shutter valve opens around 3250 rpm, and when engine load is great enough, the Secondary Shutter Valve opens to permit airflow through secondary intake passages. The engine is now using two intake pipes and two intake ports per rotor. This reduces restriction and increases airflow and torque.

Over time if you don't drive you rx8 hard so that the SSV fully opens, carbon builds up and  slowly clogs it. 

When it starts getting clogged it will start to make a medium to high pitch fast knocking noise.

To fix it you have to replace it with a new one, or clean the old one (not recommended over long term).

Here's a video that shows you how it sounds and how to replace it:

SSV knock sound video

SSV replacement video

clogged cat

The catalytic converter has accumulated so much dirt that it got clogged, letting a small amount of exhaust gasses pass. 

Buy a new CAT and replace it

Click here for aftermarket ones

OLd coils-plugs-wires

If your coils, wires or spark plugs are older than 4 or 5 years, It's a good Idea to replace them.

More power + better mpg

CLOGGED or overheating fuel pump

The fuel pump can be clogged or can overheat causing this issue. 
Check if you had a recall and get it replaced for free.

If no, have it inspected and replace if needed.