STRANGE NOISE

My 2000 Chrysler T&C van 115K mile 3, 3 L has developed a strange noise.
It is only audible when the van is cold or basically sits all night. When started it sounds like a groaning, when the motor is reved slightly it seems to disappear but when the rpm's are lowered again it's back. It also seems louder when you put it in gear. Once the vehicle is warmed you never hear it. I used the old tire iron to the ear trick and am confident its NOT the transmission, water pump or altenator, it really sounds like its coming from the intake manifold? What could this be?
Thanks for any help on this
Don