Information some may find useful.

If your Punto MK 1 gearbox is making a rumbling/whirring noise during
idling then it is almost certainly a bearing fault.  If you press the
clutch pedal in and the noise is still present, then suspect the thrust
bearing.  If you do the same and the noise disappears, then it is
almost certainly the bearings supporting the input shaft.  Pushing in
the clutch allows this shaft to slow down and stop, thus the cessation
of the noise.

This was the case with my Punto and I have just replaced the input
shaft bearings.  Thankfully, the box now sounds like new.  If you are
able to remove the box itself, then stripping the box should present no
difficulties.  You will need a bearing puller - cheap on eBay.

Once the box is removed, drain the oil and remove the selector
mechanism casing.  Then, remove the fifth gear end cover which exposes
the two nuts on the end of both gearbox shafts.  To remove the nuts,
you need to lock the box, which is done by selecting third and first at
the same time.  This can be done once the selector cover is removed.
You need a large socket (32 mm I think) to remove both nuts.  Remove
the 5th gear syncho and the other cog (extract using wedges - careful
of the teeth) which will expose both end bearings.  There is a circlip
around both - remove and take off the heavy plate.  Remove the 13 mm
nut from the side of the box - this holds the reverse shaft - and
now the casing can come away.  The whole internal assembly can be
lifted out.

What I found interesting was that the secondary shaft bearings had
integral seals and they ran smooth.  The main shaft bearings did not
and had become rough thus giving the noise.  The bearings on this shaft
are of different sizes.  The front bearing is smaller and is a standard
bearing you can get in any bearing supplier - make sure it has
plastic spacers and not metal ones.  The rear bearing is larger and has
a groove cut in the outer race for the circlip.  You can also get these
in bearing suppliers, but they may need to order them.  It was this
larger bearing causing the noise.  To remove this bearing, put the
circlip on again and pull the selector ring up against it - watch out
for the three springs that may pop out.  This allows you to put a
bearing puller onto it and extract the bearing.  I obtained a
replacement from Fiat (£20) and have noticed that it has been modified
- you guessed it - to contain side seals.  So obviously there have
been some problems with this earlier exposed design.

The box goes together in the reverse order.  It is a long job but the
parts are not that expensive even from Fiat.

Hopefully, somebody may find this of some use.