Do check the threads on the bobbing of 450 class and other helis.

I also have a HBK2 but it never had any bobbing problem. My 450PRO had that same problem. I first thought the gyro was locked too tight but then realized the tail's unwanted movement was more up-down than side-side. So the gyro gain setting din't help.

It would appear that the common cause of the bobbing is too low a head speed. Increasing my headspeed at hover nailed it.

So the easiest should be to fix your throttle curve. If your setup setup does not allow you to do that, one or a combination of pinion upgrade, lighter blades, battery upgrade etc may do the trick. Before spending on stuff, I would try first to fly with a slightly heavier load so that a higher headspeed is required to hover. If the bobbing is lessened then isufficient headspeed is the culprit.