if your spindle is getting hot or the finish sucks your bearings are more than likely shot(but not always)
spindles run between 3k and 12k depending on what type and for what machine. Cartridge spindles are cheapest for the most part. the inline direct drive ones are more expensive. if your mechanically incline yes you can rebuild them but you need to knwo what to look for also. while some think you just pop bearings in and be done, its important you know what caused the problem to begin with.
Like a crash, oil issues etc etc. bearings will run about a grand give or take for a basic spindle. keep in mind that you can destroy a grand in just a few seconds if you dont know what your doing or what to look for.
your machine shouldnt have any bearing issues as its relatively new . which leads me to believe it was cause by a crash or not having oil if its oil bearings.
as you said it just started up.
that being said if the spindle isnt getting hot. look for a belt issue pulleys loose, motor bearing going out. if its direct drive then look at the coupler, they do break down after a while.its a small rubber piece like an water impellor on a outboard if it gets sloppy or loose they will sound odd.
other things to look at is something inside the head cover.
does it change sound when you move the z up or down? that can be cause by electrical noise in the machine, current draw too much ie another problem not a spindle.
make sure your cover bolts are tight those harmonics will sound like a spindle going bad as well.
Thru spindle coolant? washers/ seals could be bad.
unless you crashed it or beat the crap out of it or if its an oil bearing it could be a host of other things.
also if your finish is worse it could be belts or pullys not ness bearings. if the spindle gets hot more than likely your bearings are going.