Small Engines (Lawn Mowers, etc.): random clunking noise, briggs and stratton, sprint engine

Question
I have a Murray mower, Model 220011X92,3.75 hp Briggs and Stratton Quantum sprint engine, with  
22-inch side discharge, which I bought new last year.
The first time I tried cutting the lawn this week, I noticed an occasional 'clunk' or "pting" noise.  I had not hit any objects, so I drained it, flipped it over to check for a loose blade.  The blade was tight and there was no obvious cause for the noise.
The mower makes this occasional noise whether moving or at a stationery idle.
It is not constant, nor frequent enough to have any pattern.  So I am baffled.  Please help!  Should I be alarmed?  Should I not use the mower?
I can't afford to take it into a repair shop, so would REALLY appreciate your help!
Thanks very much...  

Answer
Usually a noise like that, which is difficult to find, is caused by either a cracked deck or the engine mounting to the deck is loose.  Do a thorough inspection of the engine mounting and the deck.  The deck should have tell-tale signs of the blade hitting it somewhere (bare metal).  If it is either of these problems, the repair isn't too expensive as long as the deck is in reasonable shape.  Hope this helps.