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Shrill whistle emits when I release brake pedal, after about 3 miles of city driving and continues

I have always taken car to dealership for service/repairs. Recently left car at dealer service for just the above issue. They said they could not duplicate the sound. Perhaps they didn’t drive long enough…it hadn’t ‘warmed up’? Everyone who rides with me hears it. It is very very annoying.
What could this be?

Have your mechanic check the brake booster (and the line leading to it) for a vacuum leak.

Does the shrill whistle go away when you apply the brakes? If it does…it may be a worn disc brake pad. A brake pad wear indicator emits a shrill sound that’s super annoying.

I second VDC’s comment. And I will add that hopefully they inspected the brake pads? Many brake pads have wear indicator tabs that are designed to make a really annoying sound when the pads need to be replaced. This can often sounds like a shrill whistle and will go away with brake application. Yours sounds a little intermittent which wouldn’t be too odd in the early stages of this wear indicator doing its thing.

If the pads and wear indicators were checked out, then you are almost certainly looking at some kind of brake booster or check valve issue.

Make mine another vote for either the brake wear indicator or possibly the booster check valve.

Re: the dealer, it sounds like they did the “dealer thing”; They talked politely to the customer, probably wrote something meaningless on a pad, and started the engine and listened, and told the customer they could do nothing. IMHO dealerships are weird. They either take the car in and find $3000 worth of needless work that “needs to be done” or they simply don’t want to be bothered. Perhaps it’s all based on a quick look at the car. If they think they can find $3000 there, they move forward. If not, they can’t be bothered.

I’ve often wondered why, with intermittent problems which don’t happen at the shop the first try, why does the staff tell the customer they can’t duplicate the problem and offer no further help? If this same situation happened to a DIY’er, the DIY’er would come up with a series of tests done over the course of several days or even weeks to duplicate the problem. The only theory I can come up with is that shops have no way to charge for the time it takes to so this, so they just send the customers away. It seems like a better alternative is to tell the customer they can leave the car, and will be charged an hourly rate while they attempt to duplicate the problem. Maybe few customers would agree to that though.

In addition to my theory of a booster-related vacuum leak, there is also merit to the idea that the OP could be hearing the audible wear indicators embedded in the brake pads.

Personally, I think that the wear indicators sound like a cricket chirping, rather than “a shrill whistle”, but I guess that one man’s cricket could be another man’s whistle. ;-))

This car should have its entire brake system–including the booster and its check valve–checked immediately, before an accident takes place.