With regard to suspect 206 mast bearings, a little trick I learned many years ago was to run the helicopter at idle and while the mechanic was in the rear cabin with his ear close to the roof trim, get the pilot to raise the collective slightly, enough to create an upward force on the mast bearing, and then listen for a slight 'rumble' sound indicating a 'chipped' bearing. Running at 100% produces too much noise. I'd be interested to hear if this works with a 407.