I started a very long thread about this on SpeedTalk.; Many different of the top engine and cylinder head professionals in the industry chimmed in about this subject. People heavily involved in Pro Stock racing posted.
Bottom line is that no one really knows.
10 identical pro stock engines. all engines had equal power to within a few hundreths of a second with each other in the same car. 5 identical sets of headers. Engines that had the Chirp/Whistle/TGweety Bird, Lost it by changing headers. Engines receiving the headers that were Chirping on other engines that did not chirp before still did not chirp when receiving the headeres that chirped on the chirping engines. Engines that chirped quite chirping with head chenaces.
Brad and David Morgan and their creww spent much time in R&D just to pin point what was causing the strange sounds. Notta!. No conclusions.
So I guess we will all keep guessing indefinetly.
Ed