I bought a used 30% yak with da-50 on it. It has been flown for almost 3 seasons (3d flying).

The other day, and during a hover, I heard a knocking or clicking noise which I thought in the beginning some guy at the field was knocking something with a hammer but then realized it was coming from the engine. I also thought something could be loose in the frame then landed and checked the plane, and there was nothing loose. Everything is fixed firmly. I flew again, and during the hover, I heard the same sound again.


Should I be worried about this sound?

Could it be the fuel-oil mix. I have a 50:1 mix of unleaded gas/sythytic oil. I am using the same fuel which was mixed a year ago and stored inside room temperature for months. Could this be it?

I would appreciate any opinions on this issue!

Thanks in advance!