Many cheap Xformers have loosish plates, which resonate against each other, but I imagine yours are not cheap.
I cannot remember after 40 years if there are resulting forces between the coils and the core, but feel sure there are from instinct. Many are potted to prevent this and to keep moisture out. I've superglued the cheaper wall-wart types, squeezing the core plates together, but consideration needs to be given to temperature and flammability.
I take it that this is a class D power Amp?