First the facts, and only the facts :

'99 Dakota 3.9l manual trans.  119k on the clock.  Daily driver with only
work done in the last 3 months being 4 new tires.

Minor tidbits before I get to the crux of my problem.  When I'm slowing for
a stop sign or red light, my truck will idle right at 1000 RPM's until my
truck comes to a complete stop.  Once I completely stop the RPM's drop to
about 650ish which is where it should be.  It has done this consistently
for about 1 month, but didn't think much of it since it hasn't cause any
problems.  It was a little annoying, but I figured since the MIL didn't
come on it wasn't anything to stop me from using my truck like normal.  No
problems with drivability or fuel mileage.

My issue...

After about 10 miles in the truck this morning, the sound of the clutch hub
fan (I believe that's the name of the fan I hear when I first pull my truck
out of the driveway) was VERY loud.  The sound was based on RPM's, as when
I pushed in the clutch, the sound went away.  All internal gauges read
normal and no issues with drivability.  I pulled over to the side of the
road and check underneath the hood.  Nothing out of the ordinary.  
Serpentine belt was tight and I haven't heard any slipping sounds in the AM
when it is damp out.  No visible leaks or noises to draw my attention.  I
revved the engine a little while listening and could hear the fan
"spooling" up, but it didn't decrease as it normally does when I first
drive the truck. I continued down the road and again, the fan noise
increased as my RPM's increased.  The sweet spot seemed to be 1500 RPM's.  
If I kept the truck below that it ran and sounded like it normally does,
but if I went over that, there were minor changes in performance of the
truck.  There was a tapping noise (kind of like someone hitting a hollow
piece of aluminum tube with a ball peen hammer very lightly.)  It was
coming from the passenger side of the engine, but I could not pinpoint
exactly where it was coming from.  The fan continued to increase in noise
in relation to the RPM's.  Once I was near my house, I increased my RPM's
to 2000, and the fan noise continued to climb.  I didn't want to push it
and as it was I could drive 45MPH, so I got back home and drove my other
vehicle into work.

Any ideas?  I didn't get a chance to look at anything as I was already late
for work, but will be looking at it tonight.  Is there a sensor that
control's this fan and if so, what is it ?  Also, any idea what the tapping
sound was.  Any relationship with 1500 RPM's

Thanks and regards
Slick_Willy