I sprayed my flapper on my 2015 Sierra with FLUID FILM and after the first application it was half gone and after the second spray no more squeak!!!!
Had this squeak for a couple years and it was driving me crazy, didn't know where it came from.
The FLUID FILM hasn't burnt off either as i drive 120 KMs to work and back on the highway and it still is quiet and no more SQUEAK!!!
https://www.canadiantire.ca/en/pdp/fluid-film-rust-corrosion-prevention-0381567p.html
Cheers!