In addition to what everyone else mentioned, whenever you tear down the main rotor assembly make sure you check for any of the following:

DAMAGEDROTORBLADES
<ul>[*]cracks[*]splintered areas[*]dents[*]torn wrapping/ covering (on the wodden blades)[*]blade warpage[/list]BLADEBALANCE
<ul>[*]balance ofblade to blade[*]balance of blades together mounted onto spindle[*]missing or moved balancing tape[*]mounting the sameblade to the samegrip as they were each individually balanced to[/list]SPINDLEBALANCE
<ul>[*]check for equal number of spindle shims to each side of the center hub[*]make sure spindle assembly is thoroughly tightend down after replacing the spindle shaft[/list]MISCELLANEOUS
<ul>[*]pitch track[/list]Any one of these or any in combination will more than likely cause some kind of vibration.The one that some folks may not think could cause vibrations fall under the DAMAGEDROTORBLADES header Ihave above. Particularly cracks, dents, splintering or wrapping tears, if located on or around the leading edge ofa blade, can disrupt the smooth flow of air under and over the blade. This can induce an unwanted vibration and or occilation of the blades OR worse disrupt the lift the blades can potentially individually or collectively generate.

In addition, doing quick "in-the-field" fixing (e.g. taping, glueing or adding filler) ofthese direct kinds of damage to a bladecan also often affect the overall BLADEBALANCE which will then lead to induced unexpected vibrations.