Comboverfish wrote:
> You may just be paying more attention to the sound of the idle speed
> control than normal. As it opens up (revving the engine up and down
> will cause this) more air will rush through the intake tube and cause a
> sound like you describe. Unplug the ISC connector and try again. See
> if this is the noise you are chasing.

Actually I had the TPS unplugged at the time, which stops the ECU from
opening the idle control valve, so it wasn't coming into play. I'm
familiar with the hissing sound the IACV (Nissan's terminology) makes
though, and this is different. It was a short, sharp hiss that
corresponded with the suck sound from the end of the intake. I wondered
if I was just hearing air rushing inside the manifold, but when I put my
rubber listening hose up against the manifold, the sound was less audible.

I did later listen for the sound again with the TPS connected and the
IACV operating normally, and it was still there, but much less apparent.
I suspect this is because the IACV was opening and moderating the
demand for so much air all at once.

I just wish I could pinpoint where the sound is coming from a little better.