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.
Listen to attached "wav" file. The file ext was changed to TXT to allow it to be uploaded. Download and rename to .WAV and listen. It's not that noticeable and starts at about 20 seconds before the end. Does seem like when the green bars are fully filled in on the screen, it then occurs. Thanks again for all the comments.
The OP renamed it to a .txt file, so if your browser is fooled into trying to show you text, it will try to display a very large amount of mojibake on your screen. I downloaded it under a name ending with .wav and played it. The sound is totally the car's engine, doing exactly what the car's engine does when you are driving down a long hill and the battery gets full so it can't accept any more regen energy. The car is programmed to feel like ordinary cars, meaning there's a bit of resistance when your foot is fully off the go pedal. In ordinary cars, that's because the transmission is spinning the engine a bit. Most of the time, a Prius gives you the same feeling by regenerating energy slightly and storing it in the battery. But when the battery gets full, it can't do that anymore, so it continues to give you the same feeling of resistance, only now by doing it the same way ordinary cars do, by spinning the engine. If you want that to happen earlier on the way down the hill, you can shift to B and make it happen earlier. But even in D, when the battery's full, the battery's full, so the car does what it's gotta do. The reason you hear the engine rev back down when you give a little bit of go pedal is your little bit of go pedal tells the car to stop producing that slight resistance, so it doesn't need the engine braking anymore. This is absolutely normal behavior and shouldn't concern you at all. You should recognize it and smile, knowing the car is doing exactly what it's built to do. Unlike an ordinary car, where you might have to worry about overrevving the engine if you went down the wrong hill in the wrong gear, in the Prius that's not a concern. The car enforces a rev limit on the engine. It will do engine braking right up to that limit, if need be, but never more. You got a sound like a soft hum because only a bit of engine braking was happening. If you had been watching a tach, probably that wasn't above 2000 RPM or so. When stronger engine braking is happening, say 4000 to 5000 RPM, it does sound more like a roar or a whine. It still isn't anything to worry about. The car can do it all day. The engine is less stressed doing this than it is when it's firing and producing power. It sounded more like a relaxing hum in this instance because of the lower amount of engine braking needed. For more details on how the engine braking works and how to measure the amount of power it can handle, there is this thread: How much engine braking a gen 3 is capable of | PriusChat It's a gen 3 thread, so the numbers in it aren't directly right for gen 2, but there's no difference in the procedure for measuring the numbers if you wanted to do that for gen 2. (You'd have to look up some different gear ratios in the transmission.)
