And only at mid-throttle. Not at idle and not at full. At full-throttle noise from the propeller takes over. I have a canister on mine. I had the same noise from a ZDZ40 that I run several years ago. In fact I had the same canister on the ZDZ. But I have never heard that canisters give this noise. And the noise has come with age.
How to find out if it is piston slap? The bearings seem good.