Tango Oscar,
I've spoked to a Rotax service agent, and he also thinks it's the pressure relief valve. He's suggested putting a shim in to tighten it !?
I don't want to give you duff info, so if anyone thinks I'm talking bollox please feel free to shout up.
A modification that I used to uprate the performance on my oil pump (car) was to remove the the pressure relief spring and replace it with a 'stiffer' spring. The kit also came supplied with a 'thicker' than normal washer - the end result was a 'high pressure oil pump'.
Perhaps the Rotax dealer is suggesting a similar type 'fix'?
PD