The flybar maybe? Feathering spindle? Mainshaft? I'd pull the blades and run it up CAREFULLY and check for vibration. I recently found that some of my vibration during spin-up which seems to have reappeared seems to be from play between the mainshaft and mainshaft bearings. I'm not sure what the deal is, but I'm going to replace the bearings and see if it helps. After all if you think about it most of the helis weight is being held up by a 6 x 10 x 3mm bearing.