Hello, when My ECO engages on my Accord hybrid it seems like there is a misfire, it runs really rough. Also the gas mileage has suddenly fallen way off, 24mpg. What could this be?

When the ECO light comes on the engine is running on only three (some four) cylinders by use of the VCM (variable cylinder management) otherwise known as the valve pause system which is a variation of Honda's iVTEC. If the engine oil has been breaking down and sludging it can cause the system to not turn back off once it turns on and it will feel just like a misfire. It's possible that simply getting the oil and filter changed may restore sufficient flow and get the system operational again , and its equally likely that more extensive repairs may be needed. You will need a shop to investigate this to confirm or rule out the VCM system as the cause.

If you are hearing a low bass rumble when the ECO light is on this is normal. If you hear the low bass rumble when the ECO light is off then you might want to have the Active Noise Cancelling system checked. Or you can Google the instructions that explain how to disable the ANC. Replacing the ANC is expensive turning it off is free - but it will turn back on if the 12v battery is disconnected.