In last few days I have noticed faint whine/whistle while driving. The volume level changes while I am driving. (Still all of it is quite low. But everyone in the car hears it.). It seems to be coming from behind the driver. Back seat occupiers thought that it was coming from the trunk area. I have not been able to find the correlation of this noise with what the car is doing. Acceleration seems to start it. But then it stays on for some time. A few times the noise stayed for few minutes after I stopped, parked and took the key out. For a while I was thinking some problem with CV boot, but it makes noise after the car stopped, so it cant be CV boot.
I have read the suggestion that this has to do with regenerative braking and I should see battery being charged when this noise is present. I have not yet confirmed this. Plan to do it when I go out next time. However, we never heard this noise in first 5-6 years of this car. Any ideas? similar problem faced by others?

Thanks

 

Since the fuel system can have residual pressure, I'd guess that there's something emissions-related. Is there any gasoline odor anywhere around the back/underside after hearing the noise with the engine off? Also, next time the noise happens after the car is off, try to listen closely at the fuel filler door -- this could be a gas cap issue.

 

It could be one of the 3 blowers that cool the batteries under the back seat/floor area, but they should not come on unless the batteries get really hot. Try listening at the 3 vents under the front of the back seats next time, and feel for airflow.

 

Kendig, maseace, thanks for your help.

Kendig,
no gasoline odors when the noise comes up.
I walked outside the car and couldn't hear the noise. (Unfortunately I was on a street and it was too noisy, so I am going to repeat that experiment in the garage.)
Will check the fuel cap.

Maseace,
Will check for airflow from under the backseat. I have not seen these vents before.

Will update with my findings

 

I was going to suggest the traction battery cooling fans as well, but I'm not sure if those fans will continue to run when the car is off. It won't be the regenerative braking either if you hear the noise with the car off.

 

Took a while to test some of these things.
The noise is not coming from gas cap.
Also that sound just went away for few days. (I think the cold snap that we were in had something to do with it)

Now the noise is back and we can confirm that when the noise is on, the vents under the back seat are blowing air. The noise stops and the air stops in a couple seconds. So maseace's diagnosis is correct.

So next question is .. Should this be fixed right away? does it need to be fixed at all? Next time I take it to the mechanic for some brake work (in 4-6 months time), I am going to have him look at it.

rcy, the noise after car was stopped was observed only once and I have not been able to repeat that so far.

 

Those cooling fans are very important to the life of the batteries....I would get them checked...

 

Maybe the batteries are actually overheating due to a problem in a cell(s), or the temp sensor on the battery packs is malfunctioning and causing the fans to come on even when cold. There is a troubleshooting guide in the service manual for the temp sensor. I'll check if there's one for the blowers staying on also.

 

Updating after a long time .. since the noise just went away by itself for a long time.
Then it reappeared. Turns out that the battery jumpstarter that I had in the car occasionally makes that noise. It was right next to the back seats, so I kept thinking, its coming from the back seats!

 

thank you for that update, that really helps on these forums....