I rebuild the front end of my 4 last winter, installing the gussets on the internal corners. See ‘firewall wrinkle’ thread. I was amazed to have the oil canning sound from the footwell when I first taxied the aircraft. I could replicate it by bouncing the wingtip, and also just by applying light pressure to the firewall by hand. No movement on the right side, but the left was flexing. A dial indicator showed about 15 thou max movement. I put a piece of self adhesive rubber between the firewall and the engine mount. No flex.
I think the issue is a result of building some tension into the firewall structure when the two pieces are riveted up. Maybe riveting from the sides towards the middle. It’s not related to the more serious damage myself and others have had (see photos on other thread) apart from the fact that the loads on the lower corners are pretty complex, and the aircraft structure can’t quite handle them. There’s always going to be a tiny movement, which in my case, and probably yours, is just enough to tweak the built in tension of the firewall.