Here's my issue; When I start my car cold, or when I come to a stop after driving a while, there is a "spooling" sound under the hood. Kind of like a separate little motor that starts up then winds down. Again, it only happens when the engine is cold, or when I have to come to a stop after driving at least 20 minutes. I have a 1998 S70 sedan. Non turbo.

 

How many miles are on the car?

I have posted before about exactly the same noise. It seems to be more frequent now in colder weather.
I've heard it for months and it doesn't seem to have any effect on the car's operation. And, when I try to
figure it out, it goes off. In my case it is a very high-pitched sound that generally only lasts a minute or two.
My car has almost 100,000 miles and is also a 98 w/o turbo.

 

That is usually the air pump that makes the noise.

 

My car has about 112K on the engine. And it's funny you should say that about it disappearing when you try to find the source. It's gone away the past few days and it was pretty consistent before that. I'll continue to monitor it to see if there are patterns to the sound. It's been raining a few days now but I'm not sure if that has anything to do with it.

 

It is the air pump. The air pump turns on for a few minutes when the car starts to help it cycle air through and burn off excess gas left in the engine. I could here mine, I would pop the hood get out and by the time I got my hood up the sound had stopped. Really only happens when its cold, and is most likely a sign that its breaking. Mine doesnt go on at all anymore.

 

Thanks to both of you who told me it was the air pump b/c I was
a bit concerned about the noise. Now I won't worry about it at all
and presumably the pump will break soon with no consequences.