HPS vs MH Ballast Heat

I just noticed that my 400 MH ballast runs hotter and louder than my 600 hps ballast.
Is that normal? FYI the 400 mh is brand new. While the 600 has been used for about 2-3 grows.

My mh ballest is pretty quiet 400 watt it has just a slight humming

And I keep a small fan blowing on the ballest to keep it cool