Hi, here comes an update per 1. April 2014:
I was at the dealer on Saturday, but the support department was closed this day. I could try again after the wekend. Well, I then took a closer look at the problem at home. There appeared to have been a small leak below the valve at the wall, as there was a small wet-looking area on the floor below it, maybe with an area as a circle with diameter 10 cm. However, the wet surface was not water-wet, but rather a little sticky, as if oil or soap had been involved. I then disconnected the water supply hose to the washing machine. It was not very firmly tightened. I took out the strainer, which had a rubber seal ring at the circumference. All looked clean, but the rubber seal had been squeezed so that it had a few rubber threads 1-2 millimeter long ends protraining. I cut off these threads and assembled the whole thing back. Here, by the way, I discovered that the valve does not look like a ball valve, it is rather a cock-type valve. When testing, the machine gun sound does not appear any more. However, the high singing pitch tone is still there, but only when the hot water faucet is opened fully. With this I consider the problem solved. I am just amazed that the system is so sensitive to such small details.
Regards,
Leif Ernstsen