I this annoying problem and had checked all of the previous areas to isolate the problem - BUT - check the chain stay bridge. I had a track bike brought to me with an intermittent cracking when the pressure was on. It turned out that the brazing of the chain stay bridge had failed on one side only so that when the pressure was on the bridge was moving and creaking. This is often the case when a breather hole is not drilled in the bridge, hence the brass/silver does not penetrate fully. You may need to scrape the paint to check for a failure of the joint.