funky rattling/scraping noise

Hello folks, I have a 68 series 11A bugeye. i am getting a rattling/scraping noise from the engine when i am at the higher end of the rpms, it is not pre-ignition as I know what that sounds like from timing by ear. It doesn't happen consistently, nor does it last for a consistent period of time. It is most frequent when I am at about 50 MPH but as i accelerate through that closer to 60 the noise goes away but not always. I haven't been able to pinpoint where exactly it is coming from as I have not been able to re-create the sound when it is in my driveway and I am under the hood. One thing I did try, is that I went back to using fuel additive, stopped a few months ago and now two gas tanks later the noise is far less frequent and not as bad and seems to be becomming less frequent and less loud. Thoughts? Ideas? Am I crazy to think that the lead additive seems to be helping?
thanks,
Greg