Our 2008 with 92,000 miles has developed a low level hum when the car is being driven down hills, not always but often. The noise disappears after a variable amount of time, maybe 2-30 seconds, once you put your foot on the accelerator and apply power. The application of power only needs to be very slight for the noise to stop. In Dec 2024 we replaced the hybrid battery with a Green Tech Auto pack with new cells (4 year warranty). During the summer of 2025 this noise appeared. Seems like it might be related to the regeneration system since the noise appears only when the car is heading down a hill (regeneration I assume). Any thoughts on what this might be? Thank you for reading my post.
What you're hearing is most likely generator hum. Electric motors and generators do this. We used to have electric trolley buses and you would hear this all the time when they decelerated. They also did it accelerating. You might think it is a new noise, but the chances are that you just never noticed it, and now that you've noticed it, you can't "un-notice" it. It is possible that it has got louder, but there is nothing (short of a transaxle replacement) you can do about it. The transaxle has many more years of life in it yet.
Are you using shift lever "B" mode going downhill? B mode is a form of engine braking that uses the output from one motor-generator to power the other motor-generator and spin the ICE engine. It is useful in a long downhill to prevent brake overheating when your hybrid battery is fully charged with no more room for regeneration power. You can hear the ICE spinning at a pretty high RPM (hum) when this is happening.
