Just out of curiosity, I have a PH4 that has been rock solid for about three years now. I have literally flown it all over the world without a hitch. So, after about 20 hours total flight time I am starting to hear a whining metallic sound when flying. I don’t see any issues in response so far, but am a bit concerned. Is there any maintenance I should be doing on the engines? Any issues I should consider as I add flight time to these engines. Would appreciate any advice.