Small Engines (Lawn Mowers, etc.): Intermittent vibration briggs v-twin, craftsman riding lawnmower, sears store

Question
Bought a Craftsman riding lawnmower on June 27/2009.24 horsepower Briggs and Stratton v-twin. Model number 445677.  Ran and worked fine until about two weeks ago. Seems to have developed a vibration type noise in the engine. Engine usually has to warm up before the sound occurs. Vibration can be felt only on the governor arm. If I shut the engine down for about 30 seconds and restart it the sound goes away only to return in a few minutes. The warmer the engine gets the sooner the sound returns after I shut it down and restart it. Phoned my Sears store and was told to take it to the local small engine shop who handles their warranty claims. Started the engine at his shop and ran for about 5 minutes. No sound noticeable. Shut the engine off and restarted it and within a few minutes I heard the sound. Technician says he can not hear anything unusual. Tells me he's busy and can not spend any time on it until the end of the week. Today is Monday so probably won't look at it till Friday. Already two weeks behind on cutting my grass. Phoned Sears customer service and they can't do anything until the tech looks at it again. Have you ever come across this problem? I believe it has something to do with the internal governor because the vibration can only be felt on the governor arm on the outside of the engine. No vibration when you touch the engine else where. Why would the sound be intermittent? Offered to pay the shop myself to open the engine and check the governor. Says it would take him about 5 hours and cost me 300 dollars. I think he is insane. I have worked on and repaired most of my small engines and know that this job should be done in about one hour or less. Would do the work myself but the damn thing is under warranty so why should I. Any help or advice would be appreciated.

Answer
I am not familiar with his exact problem you describe, but briggs did have a problem with some internal gov gears in the past.   The history there was that the teeth would strip off and the engine would over speed.

I have a suggestion for you, You have contacted Sears and also the service techniciam who is aware of the problem.  The mower works fine with the exception of an intermittent Vibration--I would continue to use it to at least get your grass mowed then get it into the shop right after your done mowing.

If the problem gets worse you will most likely break the gear or part that is causing this vibration, its okay the engine is under warranty right?  I know this sounds very harsh, but its much easier to diagnose/fix a problem once it breaks then trying to trouble shoot the intermittent problem.  Does this make sense?  

I do think your on the right track with it being an internal gov issue however.