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Tacoma whistle while not braking

My 2003 Tacoma Pre-Runner V6 has started whistling at nearly a constant pitch which stops only when braking or after it has sat (not-running) for along period (hours). The whistle starts after the engine has been running for some time (10min +). I have been able to narrow down the whistle to the drivers side of the engine compartment but not any part in particular. I have checked the vacuum hose connected to the brake booster but is appears to be okay based on visual inspection. I have also briefly disconnected the vacuum hose (while pinching it shut) from the brake booster while the engine was running but the whistle persisted. When I stop the engine the whistle persists for about 20seconds before slowly fading and with my head under the hood I have heard a faint psst sound after it dies out. I have had not problems braking but do not want to start having braking issues. Any thoughts on troubleshooting this whistle?

I would pull the booster vacuum hose entirely and cap the port temporarily to make double sure that it isn’t that line. Another possibility is that it is the booster itself or the check valve. The thing about that is that if you have a tiny whistling leak related to the booster or check valve then it won’t cease immediately with the vacuum line removed. It would cease eventually, but that depends on how fast the leak is.

So get it whistling and then just take the booster and its line out of the picture for a while to see if it comes and goes with the booster or line. You can operate the vehicle without the booster temporarily, but only in your driveway or and empty parking lot or something. The brakes will still work, they just won’t be power brakes.

Thanks for the recommendation cigroller. Unfortunately after testing It seems to be either the booster or the check valve. The whistle died down slowly with the line removed and only came back when the vacuum hose was attached back to the booster. Thanks again.

A check valve will be cheap and if you ended up replacing the booster, I’d say to do the check valve at the same time. So I’d just toss a new check valve on it and hope for the best. If it’s still whistling - well, then a brake booster not too big a deal to replace.

Best of luck with it.