What was found
ON AFTER TAKEOFF FLOW WHEN PROPS WERE BROUGHT BACK TO 900 RPM A NOISE SIMILAR TO PROPELLER UNSYNC BEGAN AND CONTINUED. ENGINE INDICATIONS APPEARED TO BE MATCHED. NOISE DECREASED WHEN PROPS WERE BROUGHT TO 1020 ON LANDING BUT STILL OCCURED EVERY 15 SECONDS OR SO. RETURNED TO THE FIELD. LANDING UNEVENTFUL. PERFORMED GREASE LEVEL CHECK OF NR 1 & NR 2 PROPS, CHECKS GOOD. CHECKED ANVS & GVI OF TQ COMPENSATORS, CHECKS GOOD. REPLACED NR2 PEC AS PRECAUTIONARY, GROUND RUN CHECKS GOOD. CHECK FLIGHT PERFORMED AND PASSED. RETURNED TO SERVICE.