Sound after I press brake pedal and push on/off button

The other day I noticed a sound coming from my 2010 Prius (186,000 Miles). The sound is a kind of electronic whirring sound (almost kind of like a pump or something). I get into my car like I always do, put my foot on the brake, and then push the on/off button. I immediately hear the whirring sound. It’s not really all that loud and to tell you the truth I only noticed it the other day (I've driven my Prius for about 4K Miles, since August 2016). It’s a quite whirring sound that seems to come from under the hood someplace. Its a background noise of the car. I only noticed it because I just so happened to listen for weird sounds and, I didn't happen to have my music on that day. It seems to also happen when I press the brake during normal driving, albeit rather occasionally (at least that I can hear). Is this normal? Do other people out there have this happen?


Could this be the brake booster pump? Does it build up air for use in running the brakes or something? If so, I hope I don't have a brake fluid leak or something. I've not seen any real fluid leakage underneath my Prius recently (really ever, unless I've just not noticed). When I pulled out of the grocery store yesterday I thought I noticed a couple drip-drop spots, but I couldn't see any fluids or moisture residue under the car. I popped my head under the car right above the spots (rocker panels), but there was nothing. Just plastic panels and stuff. No moisture or fluid drippage. I checked the brake fluid reservoir (car was off) and it seems to be at or about the 25% level (halfway between the Min and halfway mark). Should I go buy some DOT-3 brake fluid tomorrow and top off the reservoir just to be safe? Is this whirring sound even a brake issue? Lol. I honestly don’t know if it is, I just through that out there.


I've got no warning lights present on the dash. Nothing noticeably different with the car. I just drive it every day and the car drives. The brakes stop normally. Although, the last time I was in the tire shop (not a Toyota dealer) a couple months ago they told me I only have 20% brake pad life left on one or two of my pads, and 50% on the rest. But, they actually recommended that I just get them all replaced soon since they all don't look more than 30-50% max. My Prius does seem to stop normally yet. I had planned on replacing the pads in a month or two though. Like I said, I don't really even know if thats related to this whirring sound. But, I figured I've mention it.

 

Thanks I agree. I thought it does sound pretty normal. Its not loud or anything, just a quick 5 second whirring sound, and then its done.

Its done that ever since I personally got the car.

Thank you for your help. I appreciate your post.