A progression in loudness would imply something related to wear or lubrication, rather than a single cause. Any further isolation will require hands and eyes on the bike. If you can hear it riding in all conditions then you should be able to hear it on a stand or on a trainer. You may want an assistent to pedal the bike so you can be free to look at the drive train from all angles. I would focus on where the chain goes through the pulleys first. If it's realy continuous it's not going to be one tooth on a pulley, so see if the chain comes onto and off the pulleys centered, and that the pulley is both parallel to the vertical and horizontal plane of the bike (not tilted in/out or left/right) when viewed from the rear.