my 6A (with a remote spin-on oil filter) O-320 E2D makes a noise at idle that gets louder at high idle but cannot be heard over the engine at highest power settings or in flight - sounds like a soft horn blowing. Mystified my mechanic at the time of the first flights.
I've learned to ignore it. 950 hours later everything's running as it should and the oil filter shows no sign of resigning. Noise is still there at startup and taxi, though. Might be something to this; oil filter plumbing might be the source.