'99 1500 5.9L - hissing & rough idle a few minutes afer startup - with Video

[sevenHD]

Startup is normal & smooth for first few minutes. Then a hissing sound starts ( constant, not pulsing ) seems from passenger side of engine, and engine idles rough-ish ( runs rough-ish too ). ODB scan tool has no codes to show, no check engine light either.

? something vacuum related, PCV valve ?

Attempt at video link here:
https://photos.app.goo.gl/dqfam5dRiengvn9W8

seems like clicking on the video on google photos page will open it full screen with audio.

 

[RodeoRam]

My first guess is the air filter housing not tight on the throttle body. That used to get me almost every time I had it off, but it would run rough right at startup and throw a code.

 

[Yeret]

I used to have this happen somewhat frequently during cold starts in the morning, especially if it was humid for whatever reason. Like in your case, the idle quality would get choppy during this time.

This was also during the time when my intake plenum gasket was shot and the engine was taking in oil as long as it ran. Years following the plenum gasket replacement, I've never encountered it again.

If I were to theorize, the ingested oil was fouling the air/fuel mixture, resulting in a loss in power or misfire, which then caused the idle RPM to drop, which then caused computer to open the intake air control valve to compensate. The hissing noise would therefore simply be the increased intake through this valve.