I just noticed, although it may have been going on for some time, a short (about 2 seconds long) high-pitched whistle whenever I turn off my engine. There is no perceptible noise when starting up or running - just when turning the engine off. The noise is definitely coming from the engine compartment. If I listen with the bonnet up I hear a hissssssss and then the whistle after I turn the key to shut the engine off.

Any ideas? Belt, maybe?

[I have a 15% pulley and a 2% crankshaft pulley installed. They have been in for almost a year. I don't recall hearing the whistle immediately after the install.]

 

Sounds like a vacuum-induced whistle. Or, perhaps the the a/c compressor. Since the engine is stopped, it's not likely to be a belt.