Noise when decelerating and at startup?

[Joe1911]

I have an 02 7.3L PSD.

The other day, it started making this strange "whirring noise". It is audible at start up, then seems to fade away. When decelerating, especially at speeds greater then 20MPH, it is VERY audible. It makes the noise regardless of whether I am braking or coasting.

It does not appear to make the noise while it is in neutral, only when in drive or reverse. The noise sounds like it's coming from the drivers side of the truck, as it is harder to hear from the passenger seat.

Any help in resolving this would be much appreciated!

[Joe1911]

Some further investigation revealed:

The noise is heard at Idle, however if the accelerator is pressed, the noise immediately goes away. If the Engine RPM's remain above 1000, the noise is not present. Only is audible when 1000 or less RPMs.

I popped the hood and let it idle. The sound appears to be coming from the middle of the truck. Sounds like perhaps the fan clutch?

Does this sound possible?