Mine was also noisier in one direction (left). It is the nosiest when I don't connect it to the PC or the walloutlet. I opened it to see how it was build up and I must say that it looks strong and solid. I checked the screws, but they all looked good. I also don't think its the screws, it is probably the motors or the gearing in between..
The funny thing is when you disconnect one or both the motors and you turn the wheel.. (when it isn't connected to anything) Then the wheel is very light to turn.. This means that when the G25 isn't connected, the motors work like generators (logical) and are 'braked' by the (passive) electronics..
So when you turn the (disconnected) wheel, the resistance that you feel are mostly the motors that can't turn freely and that will probably cause resistance in the cog's and might resonate.. I think that is what I hear with my wheel..