Suddenly my engine started making a loud
chirp chirp chirp noise when I apply the brakes at low speeds. It sounds kinda like an old Chevy van with bad power steering and I'm hoping someone has heard this before and can direct me to the source. It's an odd one and very difficult to track because it stops shortly after the car stops.
It seems to occur when:
- Car is stopping or recently stopped. Once I was able to open the hood and look/listen to the last few chirps but usually it stops within a couple seconds of stopping the car.
- Brakes are used. Even the parking brake causes it so it seems unrelated to vacuum boost. Coasting to a stop by going uphill simply slowing with engine braking does not trigger it. Downhill stops with firm braking are most aggravating. I can even make it chirp louder or softer by varying brake pressure.
- Engine has just revved. It need not be in gear but it does need to have revved just prior to stopping. So for example I can coast downhill in neutral, tap the gas, then pull the handbrake and it will begin chirping.
It seems too slow to be the radiator fan and occurs independently of the A/C or steering. Could it be a squeaky diverter valve? But why would the valve be moving around so much when the engine is idling smoothly in park? The one time I got the hood open it just sounded like it was coming from the middle of the engine bay somewhere. Happens hot or cold, but only a few times per week so it's very difficult to reproduce consistently.
Update: Cause was found to be crankcase pressure regulating diaphragm integrated within the valve cover. See here for details: http://www.e90post.com/forums/showthread.php?t=1197666
Attached videos say it all. Remove the .pdf extension. The driving video illustrates the front main seal chirping like a dolphin whenever I hit the brakes. The under-hood video shows the crankcase vacuum reaching -1.5psi (-105 mbar) as the rear main seal squeals and silences in response to the oil cap being opened.