Was flying my friend’s Turbo 182T, over the coast out of Tamarindo Costa Rica, 3rd flight , of this new engine, after climbing to 3,000 ft and reducing power I could hear some noice coming from inside the cowling, also plane felt very slow, and I saw the rpms dropped to 2100 when I reduced power and the prop pitch was all the way forward.
Increased MP to 23 and rpms stayed at 2150 at full prop pitch, increased to 24 still the same when I increased to 28 MP rpms went to 2200, no more testing and time to land.

Went back to the airport and landed.

Tomorrow we will open the cowl and check and have a mechanic look at it.

Any ideas what can cause this?