ST2 standard is 15/42, ST4 is 15/43 and ST4S is 15/38
I run 15/43 on the 4S and it's great.....
As Andy suggests, up a bit on the rear would help with a 15 front... you'll find that the whine is the chain rubbing on the upper chain buffer on the top side of the swingarm at the pivot end.
It gets worse if you add a bit of ride height too.
The 15 tooth front should get rid of the problem for you.