[L473ncy]

Maybe the lower is too small and making the gearbox shell bow in? Try spikes solution and if it doesn't make that sound then you can hypothesize that it has something to do with the lower and gearbox shell fitment. Either shave the outside of the shell or shave the inside of the lower. However that said, I've never heard of this problem specifically before.

I know that VFC shells don't fit into all guns (I actually needed to shave down a lot of the exterior of the VFC shell to fit it into a Madbull lower, namely that bolt release spring assy. thing and the small "post" that juts out of the shell).