I had the same problem and neither I nor the shop could get it sorted out. The same symptoms and the same noise noise and it progressed to about the same point as yours has and at about the same rate mileage-wise. One day out of pure frustration I decided to lube one part per day until such time as I had lubed the entire drive train. The first item on the list was the left pedal bearings. One little squirt of Tri-Flow and the noise was gone and has not returned.

Who'ld have thunk? It was quiet on the stand and spun quietly when not being ridden, but when in use the bearings were just dry enough to sound out!!

Good luck with yours and Happy Trails