Just noticed this today on the DR-1. When I throttle up to about 40-50%, I can hear a quiet grinding sound. It sounds almost like chattering servos (like back in the day when you used to get interference on radios). I am thinking maybe the main bearings? The thing only has 5 - 8 packs through it. But, the rotors are a bit chipped up. It's my first heli.
Is it hard to change the bearings? How many are there in the heli? Two come in a pack.
Check your main gear i stripped mine and it was grinding pretty bad. Its a easy fix.