I had a huge coolant loss on my bike, after some time, it was getting even worse (I was loosing approximately 1dcl of coolant per 1mh). The leakage through the hole behind the water pump was not so scary in the beginning, but later I was loosing a huge amount of mixture of coolant and oil. Although I did not observe any decrease in oil level.
First, I changed the seal under the head, did not help.
Then I have bought new radiator cap with stronger spring, that did not help as well.
Finally, I decided to reseal the water pump. I discovered that the special atypical SKF bearing was worn (too loose). I think, this was the reason why my motor was whistling. (Some say, it is the flywheel of E-starter which whistles, but I have the starter and all related wheels put away and I was whistling!!! So?!).
It is important to change the special bearing (in one piece with its special coin-shaped seal), the small O-ring under the bearing and the small water seal at once (has not to be original, u can buy such seal anywhere for almost free).
Why to change everything on the shaft at once? Because if there is a worn bearing on the shaft, there is more room for movement of the shaft on its end where the water seal is and this causes leakage even if the water seal is brand new. Not changing the bearing would destroy the water seal in a very short time, anyway. To be sure, that you will not ruin your new water seal while installing it on the shaft, use oil lubricated smallest finger cut off from a thin latex glove which you put on the shaft and remove it after installing the water seal. To prolong the life of the water seal and the shaft do not mix the coolant with water, or mix it max. 50:50. In my case, it was not necessary to install two water seals, one does it.
RESULT:
My bike does not whistle anymore, there is not even a drop under the water pump and the level of the coolant does not move anymore, absolutely. Till now I had to inspect the lever almost at every stop and a supply of extra coolant in backpack was a must.
If somebody says to u in future, "dont worry, 9 of 10 husabergs have leaky water pump" (Fero Hostinsky, for example), do not calm down hearing this and do not hesitate to reseal the water pump immediately (do not forget to change everything on the shaft!!!). Its worth the effort, u will see after u will have it fixed.