Frmmli

My 2014 ES 300H at 55,000 miles and prior did not have the burping sound and the sound was noticed around 60,000. It come on under moderate speed and moderatelyhard braking usually right at the end of the braking stroke or shortly after. I will not call it intermittent but rather at the right combination of speed and braking effort. The sound is like hydraulic fluid or air going through a restriction/orifice, like a noticeable 'urrrrp' burp that is definitely noticeable. No pulsation to indicate activation of anti-lock brake. It is none of those normal stuffs/complaints like dry bushing, rubber boot, related to the pedal or linkage. One dealer said that is related to the braking system taking over suddenly from the Hybrid re-generation system switching out and the brake fluid rushing in. I do not buy that fully. I am also wondering is it started to happen after the 60,000 miles maintanance which include a replacement of brake fluid.

In subsequent searching, I found this exchanges in a Prius forum:
Weird burping noise when braking
Discussion in 'Gen 3 Prius Main Forum'

- My 2010 Prius has been making a weird grunt or burp when I stop. I thought it was the anti-lock brakes being oversensitive, but it happens so often now that I can't believe that's true. I live in San Diego and it is NOT wet here. I mentioned this problem when I had my first oil change and the dealer said they had never heard of this issue.
- The noise goes away sometimes, but comes back if I do anything other than come to a gentle, slow stop. Now it has started happening even when I am applying the brakes while cornering. It has also changed from a single grunt to a couple of coughing sounds sometimes.
That alarmed me, so when I took it in for its 10K service. I asked the dealer to test drive it. They can't have actually done the test drive because I was easily able to produce the noise again on the way home!! They claimed that the noise was due to a nut on one of my wheels that fell off because they failed to tighten it properly when they rotated my tires. NOT. It has been intermittently making this noise since it had less than 1000 miles.

- The Brake "Bark" is a known problem in 2004-2007 Prii (do a search). There was a TSB (TSB BR001-07) covering it. The noise is generated by air bubbles in the hydraulic fluid going through the brake "Actuator" that is mounted on the firewall (passenger side). It was eliminated in my 2004 by bleeding the passenger front brake, but in some cases (where the actuator is letting air in) you need a new actuator (???).
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Freds430

Same thing happened to my 2017 RX 450h after some brake work was done. Took it back with same sound and they had to bleed the break line. It had a tiny bubble. I actually recorded the sound on my phone and played it t for the Service Manager who immediately knew what it was.

Frmmli

Thanks Freds430. Glad to have the shared experience to show my two local dealers, both have made it the mission of the service advisers to tell me nothing is wrong and the brake is fine, and it is normal sound of fluid rushing in to take over from regen braking in a hybrid!

gamma742

This is happening occasionally on my wife's CT200h. I think it's time to have the Brake Fluid changed. She has over 100K miles on hers.