It's almost like the commercial. Describe the noise to us.
Try this run the engine fast and then hit the stop. While in the stop position what noise do you hear? If the noise you were hearing is gone then somewhere in the engine. If not then something rubbing.
Maybe time for a new muffler.

Could be anything from a worn connecting rod bearing to a loose blade. If it's a bearing,you won't be hearing it for long. Check oil level, and change oil to SAE30HD to see if it quietens it. Zone 10 is hot, and 30W might be needed for your well-broken-in mower.
Check engine mounts and blade bolt. Check the blade driver after making sure the blade bolt is tight - try to rock the blade by grabbing both ends to see if the driver is loose on the crankshaft. This might also show excessive play in the lower bearing. While you're at it, see if the lower crank oil seal area is leaking. Check all fasteners you can get to on engine, along with spark plug do not overtighten anything, esp. the plug. Check cylinder head bolts, intake manifold and muffler attachment. Like rm said, might just be the muffler.
A variation on the trick rm posted is to run it fast, then slowly ease off the engine control bail until it cuts the ignition off but doesn't put on the engine brake. This gives you a longer coast-down to listen to. A bad bearing might be quiet on a cold engine, but will get louder as it gets hot/speeds up. Under some circumstances, you may not hear the knock on the coast-down, only under a load.
You can also remove the spark plug, tie the bail down so you can spin the blade, stand the mower on it's side with carburetor UP, and move the blade quickly back and forth to detect any play, which could be at the connecting rod end or the wrist pin in the piston.