I am putting on an APC 16x8 prop on an Eflite Albatros. Motor is a Dymond 2832 (similar to eflite power 32) and speed control is Dymond 60a with 5a BEC. Battery is zippy 14.8v 3000mah. Vibration noise only commence when i am at slightly below half throttle. whan i am past half throttle vibration noise disappears. i have already balanced the prop but have not yet balance the spinner or the backplate. i thought that if something is out of balance it would shake and vibrate throughout the range of rpm and NOT at just slightly below half throttle and vibration disappears at any other range. any help on this matter would be much appreciated.

I am guessing that you are hitting a resonant frequency of some sort. All structures will vibrate at a certain frequency (think of a tuning fork). If you have an out of balance prop spinning at the same RPM as the natural (or resonant) frequency of your airframe, the vibration will be amplified. Even if the prop is balanced, your airframe will tend to vibrate more at that speed.

This behavior is also found in full scale aircraft which is why aircraft often have certain engine operating speeds that they must avoid or pass through as quickly as possible (I started out my career - more years ago than I care to admit - working for Hamilton Standard on aircraft designing/testing full scale propeller systems).

You could try soft mounts to isolate/damp the vibration, but definitely avoid prolonged operation at that particular speed.

Al

And get the spinner and back plate balanced.
You can like said go into and out of a balance/resonance problem by speed.