Here is a trick that has worked for me in the past. You need an indoor trainer, buddie and a paper tube from a roll of paper towels. Ride the bike on the trainer and hopefully you will duplicate the sound. Have the buddie use the towel tube to listen as to where the sound is really coming from. You may be very surprised to the real source of the noise. Think of the can and string thing you did in grade school science class.

If you get the sound when the bike is on the a stand then you don't need the buddie. Good luck.