It sounds to me like you may just have a brush roll with bad bearings.  Either that, or one of the end caps isn't sitting in the carrier correctly.  Brush roll issues can make a real racket sometimes and you can think something horrible is wrong with the vacuum.  A quick way to tell if it's a brush roll issue is to use the belt lifter to lift the belt off the fan pulley.  Then turn vacuum on and see how it sounds.  If it's making the same noise, then you know it's not the brush roll but something else.  But if you determine that it's a brush roll issue, remove the brush roll and examine the the two carriers.  Sometimes the lugs get spread out and the end caps don't sit in their tightly enough.  That will make a terrible racket, because the end cap will just rattle around.  If that's not the case, then you may just need a new brush roll.  As for fan case sealant, I've always just clear silicone calk.  If you don't use something, it will leak air.  Hope this helps!

Lexan is the material that Kirby once used in their fans.  The fans today are made out of a material similar to Kevlar.  I think that's why people say that.  If the brush roll isn't the culprit and it still sounds like a thrashing machine, then you probably need a new fan.  Sometimes it's hard to see the broken piece(s) and even nicks that catch pet hair can throw it off balance.  It might be a good idea to stop running it until the fan is changed, because it will run the armature off balance cause the the armature or the field (or both) to short. 

The fans themselves are exactly the same; however, the certain items in the kits are different (depending on the model).  Just look for the fan kit that's specific for your model.