Based on what you have said, I would suspect a resonance which you could confirm with a bump test. In the interim I would be looking for something that might have significantly changed the mounting base stiffness. That could be something as simple as loose bolts to something as complex as a cracked weld. But since mass hasn’t likely changed then you have to look at a stiffness change that has caused the resonance to likely shift downward. Check piping supports as well. Basically look at the entire system; if something exists to position and hold something rigidly, check it to make sure it is doing its job.