Yes Groestl algo i have no problem.
Skein algo tested on suprnova, miningpoolhub and now zpool have all the same problems : hashrate betwenn -30% and -10%. Average on 12h on any of these pools is 11,8GH instead of 14GH

The option is baikal-fan
For Skein i set 10% and it remains below 30 degrees. It is still a little loud.
50% and 100% are almost as loud so i think thats why you hear no difference.

The main problem i have is the average hashrate reported on suprnova for G28 is 20% less than it should be (with high fluctuations it can goes from -40% to -10% averaged on 5 units). Whereas with X10 still on suprnova the average hashrate is 100% as advertised (fluctuations are small 5% with 5 units). this is for skein algo.
Anyone had the same problem ?