“a rubber bicycle horn sound to a chainsaw sound” I am having a little trouble imagining the sound. Can you try a little more description?
Type/age of vehicle?
I cannot imagine why a fuel pump would make noise when you take your foot off the gas. The most common noise when you take your foot off the gas is a heat shield vibration, which is usually a high-pitched metalic chatter. The second most common noise when you take your foot off the gas is a vacuum leak whistle.