When you clutch the blade, the engine loses the blade's flywheel effect at the same time the governor lets the throttle close a little while maintaining the governor RPM setting, probably around 3000 RPM. You might expect the engine to sound different at this time. I think it may have a sticky valve, but its not really a problem since the engine responds to the governor and runs properly under a load. You could try using a mild solvent in the oil and gas to see if that helps. Get some Marvel Mystery Oil at Wal Mart or any auto parts/supply. Put 1 oz in the gas tank and 1 oz in the oil. The measure doesn't have to be precise - a little more won't hurt anything. Give it about two or three cuttings to see if it makes any difference.

Thanks for your input. I put some MMO in the oil and gas but haven't really used the mower to cut the grass yet.