well.....
1. Dunno. Double check its not summat dancing around inside your frame........ also, off the bike, spin the wheel really, really slowly and have a listen. You might just be flinging it too fast too allow the thing to rattle?
2. Seat tight on the post do you mean? what sort of fixing is it? if its conventional guts, then they are just shagged - end of. new guts will cure it.
3. You can run some light oil into a conventional freewheel (lay the bike on its side and drip oil into it via the small gap between inner bit and outer bit), that will shut it up with no adverse effects. DO NOT do this with cassette hubs, they do not like oil or grease at all and will kill you eventually if you have them running silently.
4. brake quality is more about clean braking surfaces, deglazed blocks (rub em on a bit of sandpaper or the concrete floor), and a nicely lubed cable routed correctly. I used to run mx1000s on Tuffs with no worries
Make sure you actually have a gap between block and rim, so the wheel runs free, this will actually make the brake better
Hope that helps
