Please help identify source of engine noise

Its your brake booster pump. Seems louder than it needs to be. I can hear mine sitting in the driver seat. Always comes on when I open the driver's door and will cycle on again after using the brakes a little bit. Its normal. What is not normal is if it cycles on when you are not using the brakes. That is a pretty good indication that there is a leak

I swapped out my master cylinder (accumulator?) and it definitely helped, but I have read of others that had the cycling going on for years.

 

Monitor how often you hear the noise... Once you hear it more than once in a minute its time for a brake booster pump swap.

 

houston, all systems normal

 

I normally hear this sounds whenever I am coming to a stop (using brakes of course) and it goes away as soon as I am fully stopped. Perhaps it's not the same sound, but I definitely do not get it when the vehicle is stationary, unless the car is not on and I open the driver's door.

 

Mine has been cycling at 22 seconds for several years.....

 

So no brake fluid leaks anywhere? Reservoir is good? All pads look okay? No check engine lights? Brakes are working nominal?

If everything is good, I'd not start a "parts canon" just yet but I would do a good brake fluid drain and fill if it hasn't had one in 4-years or more.

 

A brake flush won't fix it for more than a day.

 

You don't think it may be the root cause? I don't know what sensor(s) the brake booster uses, but do know brake fluid can start to get pretty dark after 4-years. (And, doing brake jobs, I've learned to always open the brake fluid drain plug before compressing the piston, fluid in there is always pretty black, even if the overall brake fluid looks good.)

 

How often is it cycling?
If you are going to do one, do the other at the same time.
Because if one doesn't fix the issue, you'll have to go through the whole job, again.
Unless you like doing it.

Did you diagnose it to verify the issue, if it is an issue?

 

Side story...on Saturday, my wife told me she went into the garage and her Corolla Hybrid was making all kinds of noises....even though it was hadn't been used and was turned off.
So I started with my hybrid speech about the high-tech fuel evap system and noticed her eyes glazing over so just said, "It's normal....all Toyota Hybrids will do that....that's why you don't want to ever leave a hybrid undriven for a long amount of time...the 12-volt battery will get low."

 

A normal brake flush is always a good idea every 50k-100k depending on the climate, humid or dry.

Rapid cycling of the brake booster assembly/master cylinder is a sign of an internal hydraulic pressure leak.

It could stay that way for months or even years but usually will code when it takes ~90 seconds or more to build pressure. Coding does not mean loss of function quite yet but is an imminent failure warning.

Experience has shown a complete "replacement" mode brake flush will help for a day or so, not the greatest return for the effort.

Virtually all gen3 hatchbacks and v wagons were covered for 10 years/150k because of frequent problems with the brake booster assembly. The Toyota coverage has expired with the possible exception of low mile 2016-17 Prius v's.

Attached is a brake booster assembly pdf which includes this statement:

Toyota has received reports about certain internal malfunctions of the brake booster assemblies...

One or more of the following specific Diagnostic Trouble Codes (DTCs) C1391, C1252, C1256, or C1253 will be stored in the vehicles memory to be used as verification.