Small Engines (Lawn Mowers, etc.): Briggs 675 knock, intake smoke, castrol syntec, briggs stratton

Question
Hello Mr. Dunn, I hope you can help me. In May of 2008, we purchased a Craftsman 22" Self Propelled lawnmower, with a Briggs & Stratton 675 series flat-head engine. Worked great for the first year, I changed the FF oil at 5 hours with Pennzoil YB 10W-30 conventional.  Worked great all season (About a 35 hour season, and about 10 this year).

Well, after being stored up for the winter, when mowing now it has a knock.  Under no-load idle (one speed, no throttle control) it nearly vanishes, but under load when cutting grass, it is definitely a knock.  Have not heard it from a Briggs 500 series, Briggs Quantum XM 5HP, or Tecumseh 4.5HP. It isn't metallic sounding, almost sounds like two pieces of wood knocking together lightly.  I changed the oil a month ago with Castrol Syntec Synthetic Blend 10W-40, and the noise hasn't gone away.  And it also smokes from the intake very thinly when running, the light has to hit it just right to see it.  But when you shut it down, and start it up immediately, it puffs white smoke out the intake filter.  

What could be causing these problems? It gives no hints of problems running, always starts 1st pull, runs strong. Replaced the spark plug this year with a E3 plug, but it was doing this prior.  Also, no evidence of oil fouling on the spark plug, and I have triple checked that it isn't overfull. Is this characteristic of Briggs 675 series engines?  Also what is the actual Horsepower rating of this particular engine, the torque rating is 6.75, and I'm hearing the actual HP is 4.5-5.  Any help you can provide would be appreciated!  Thanks in advance!

Answer
Hi Nicholas
 Your knock could be several things. It could be wear on the piston rod and cap on the crankshaft journal, cylinder wear, wrist pin on the piston, etc. Also check the blade for tightness and wear and also the flywheel key for any distortion. These engines were designed to use 30 wt oil and not synthetic. The smoke from the intake is from the crankcase breather tube and should not be a problem. I would suggest to change the oil with a 30wt non-detergent oil. When changing, look for any metal shavings. I'm not sure of the actual hp of the engine but 4.5 - 5 hp is about right. Hope this helps.