Instead of grease ,try plumper 's tape on the treads of the seal BB . Try replacing the left crank arm , maybe the hole the axle goes into been enlarge , I know most will tell you not to put grease on the BB 's axle where the crank go , but try it anyway , it wouldn't hurt anything plus it might quite the noise . It might just be flexing of the frame ,but if it was you would notice that before now .