Here is an interesting mechanical problem from years ago. My 97 year old uncle used to be a die hard Pontiac man and owned several during the 50'* - 70'* and I remember a couple late 60'* early 70'* Catalinas with 400'* that would scream. His Catalina was making a strange whistling sound and he had several shops take a look and nobody could find the problem. Most mechanics went straight to the vacuum hoses and searched high and low for anyplace that air could escape and they would eventually throw up their hands and send him on his way.
He finally found the right mechanic that had seen the problem before and he put the car on a lift and took a Sawsall (not sure what they called a Sawsall in 1970
) and cut a 2 foot section of pipe out of the exhaust. The car had double wall exhaust pipe and the inner wall had seperated from the outer wall and it collapsed inward and had closed off most of the pipe causing the whistling noise. The outer wall was uneffected and from the outside it fine!