Whistle under the hood

It is a little bit colder these days in montreal...
This morning I started the 1998 Rodeo (3.2/ Automatic/ 50K miles) and between 1900 and 3000 Rpm I hear a whistle comming from the engine. It cleared away as the engine warmed up. I did not push it at all so I can't tell if there is a power loss or not. First day it does that.

Is that the first sign of a leaking intake gasket?
Could it be a stupid idller that starts to rock not roll?
Anybody has ever seen this before?
Should I simply raise the volume on the radio or leave the truck in the driveway for a week to punish it?

Thanks