I think it's more likely it's just the tank flexing because of a vent issue.
Manufacturers are notorious for not installing the roof vents correctly. If you go up and remove the lid of the vent, you should find that there's space between the top of the black pipe and the under side of that lid. Often there isn't enough gap and the vent becomes useless. If that's the case just use a hack saw to remove what ever excess you can. In the past I've just cut some deep groves in the pipe to fix it. It doesn't have to be pretty in order to work right.