Loud sucking noise?

So for the last month or two I have noticed a rather loud humming fan type sound coming from my engine bay when I first start my Blazer up in the mourning or after it has been sitting for a long time. The sound goes away after 20sec or so. and then it stays off. I opened the hood to see if I could see anything when the sound was going my fan wasn't running so its not that. Today I started it and the same old humming noise turned on, but this time there was a sucking sound also. It sounded like a straw sucking out of an empty cup. And just like every other time after about 20 sec of me freaking out, it went away. Any ideas? I would really like to nip this in the butt before I am left stranded.

I too have had the humming sound, which goes away after I start driving. Don't have a clue what it is. The sucking sound may be air in the system with your heater core. I've had a sucking sound come from there, and it also sounds like it's bubbling too. Maybe check there?

Sounds like it could possibly be your A.I.R. pump, though that normally runs when you first start it up until the engine gets warm.

I get the humming sometimes, but not the sucking noise. I've been assuming it was the fan clutch and was just waiting for a little warmer weather before I replaced it.

I also have that sucking noise when I first start my 95 Jimmy when it hasn't been run in a day or normally over night.  I first thought it was the fan aswell because when I started driving it, half way through first gear the sound would go away! I find it even takes a bit of hp away. I then started thinking it was the tranny but don't get frecked out because it's not! Check the cluches out on the motor and see if anything disengages after the 20 seconds. It might lead back to whats making the noise.
 
Marc   

So what I have gathered from this , is that many people hear the same thing that I do, but nobody knows what the hell it is. HMMM. I am surprised that Kyle hasn't chimed in with our answer yet. hint hint.

Sorry folks, my profile and message notification still isn't working properly and I'm having to do review posts in a very different way than I have been doing for the past 3 years... 

If you think it may be the fan clutch, remove it and see...  If you think it may be the air pump, disconnect the electrical connector to it and see if it changes.  If either of those things doesn't change the noise you are experiencing, try narrowing it down with a mechanics stethoscope.  In a pinch, you can use a long screw driver, putting the handle to your ear.