Hissing sound when turning propeller

Sounds a bit like a leaky exhaust valve. I would start diagnosis with a leak down test and a bore scope.

I agree with Murray. Start with a leak down compression test. Not a static test. A leak down set up with the two gauges.

I’d be surprised if this turns out to be a timing issue. Leak down and borescope make perfect sense.

Did this hissing sound develop all of a sudden or did it “sneak in” over time? Do you experience any loss of power, like not reaching the normal max. revs or so?

Thank you all for swift replies! I will forward them to the other members of our owner group, who have more experience with Rotax motors than me. As for how long the sound has been there: I do not remember, as it was pointed out at the yearly control, and I was not aware that is was an anomaly. Part of the story is also that the previous owner had stored it in a hangar for two years with no engine start, only propeller turning from time to time. As for loss of power: I will forward this to the rest of the owner group, who have more experience with what to expect. Thanks again.

Chapter 12-20-00 (Page 7+) of the ROTAX Maintenance Manual - Line for the 912ULS describes the differential pressure check and compression check procedures.   https://www.rotax-owner.com/en/support-topmenu/engine-manuals#current-engines