Made a little more progress last night.
Replaced the muffler with a good known muffler. I also replaced the muffler gasket and still the same sound.
I continued to feel around the base of the head and pretty much confirmed the engine has a blown head gasket. I used mark�s suggestion of the soapy water (from underneath my bbq) and confirmed air was escaping out the head. You could also feel it quite considerably with the muffler removed. I have definitely found that strange sound.
Tonight I plan on removing the valves and pushrods to access the head gasket. I have a spare head gasket available from the GXV140 engine which the Sanli is a clone of so it should be sufficient.
What I am worried about in this instance is the cause of the blown head gasket. Could this just be poor quality control at the factory or could there be another specific cause of the problem.
I have not come across this problem yet on relatively new OHV mower engines and want to make sure my bases are covered.
