Check your strut bearings. Located inside strut, copper housing with rubber inside. Usually two pieces(at least that is how the ones I replaced were), one at top and one at bottom. Once they wear out, your shaft starts rubbing against strut ( metal to metal )and can cause some noise. Mine were bad and would hear loud "pop" evry now and then. You can usually tell by looking at them if they are worn evenly, if they are worn unevenly it could be an alignment problem. You will have to remove prop, and shaft to replace and can correct alignment problem when putting back together.
www.skidim.com Call Vince or Richard, they sell bearings and can walk you through it. They also might be aware of other causes, very helpful guys. It is not a big deal provided you have right tools.