OK, I went to a junk yard and pulled a couple of speedometers, one from
a Ranger and one from a Bronco. They were both older than '94, and had
one less digit in the odometer. I also found another slight difference,
the holes on each corner that the Tach and other gauges index off of
were smaller, so I had to drill and use nippers to make that fit. The
face was also a little different. Unfortunately there were no newer
vehicles. I probably should have bought the correct speedo on Ebay, I
could have gotten one for about $45 with shipping. However it might
have had the same problem...

Quick and dirty speedo removal process: Remove the trim around the
instrument cluster. I used a claw hammer. Then remove the screws
holding the clear bezel and the black spacer between that and the
instrument faces. I used a torx bit on an electric screwdriver with a
12" extension.

Pop out the tach and the gauges starting from the outer ones, just pry
gently. The Speedo is held in by them. If it's an automatic, you'll
have to deal with the cable that moves the gear shift indicator. I just
cut the cable with wire cutters, or you can remove the torx screws
holding it.

Then reach down by the gas pedal and pull up on the speedo cable to get
some slack, and gently pry out the speedo until you can get a hand
behind it. Push on the flat on the white plastic catch to separate the
cable from the speedo. You don't want to squeeze on both sides of that,
just push on the flat with a finger while gently pulling on the speedo.
Squeezing both sides won't allow it to unlatch. That's it.

The Ranger speedo was dusty and had wear marks indicating that the
spinning magnets were rubbing on the needle cup, but the Bronco speedo
was in pretty good shape. The needle had very little side play. So I
bought that for $17.

I reset the odometer to match my old one by driving out a pin that held
a series of gears that link the digits of the odo. I slid that pin out,
and then the digits were free to turn. I set it to 63999.9, reinstalled
the pin and gears, and spun the input with my fingers to verify that it
went to 64000.0.

I installed it using my quick and dirty procedure, and it fixed the
problem.

Next I took my old one apart- hold the cup while twisting and pulling
on the needle, and it comes off. Two screws hold the face on, then two
more screws hold the front of the meter to the frame. The needle and
odo assy comes off, leaving the frame and input section.

The spinning part has a hole in the end, which is a bushing for the end
of the speedo needle assy. That was worn, allowing that end of the
needle to flop around, making the rattling noise.

So that's the cause- a worn bushing.
-Paul