Vibration is a common problem that can be caused by many factors.  Most are from the rotor assembly.  Here is a checklist:

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A common problem with RC helicopters is vibration. Excessive vibration makes hovering more difficult, uses up power, causes "gyro drift" and increases wear and tear. Several things can cause vibration, which makes it difficult to identify its source. Below are some of the possible causes:</p><ul>[*]Check the rotor core and look for cracks in the plastic escpecially on the ears were the two bearings hold the rotor blades.  This is usually the culprit.[*]Use the same blades for replacement. [*]Look for small cracks or chips on the blades.  This can cause  vibration as one blade is cracked versus the other blade.[*]Crooked rotor blades. The blades should be at right angles to the fly bar.[*]Rotor blades aren't tracking. They should both travel in the same plane.[*]Rotor blades too tight. If the blades are too tight, they can't self-adjust from centrifugal force.[*]Rotor blades are too loose. Adjust the blade grip screws for these two problems.[*]Main gear missing teeth.[*]Flybar paddles misaligned. The paddles should be nearly parallel to the swash plate, preferably 0 degrees.[*]Make sure the rods from the skids are fully seated and the rubber sleeves snugged up to keep them from shifting. I added a rubber band to mine to keep the skids snug.[*]Finally, you might to check if the main gear shaft is bent.  You will have to disassemble the rotor assembly to get to the shaft.[/list]
Hope this helps.  You will be a mechanic in trainning after this.
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