Internal Engine Considerations at high milage

[Canuck]

Hey as some of you frequent posters know.. my 2003 GZ is at 50,000 kms

The Oil has been changed (10w40 Suzuki)
valves are within specification
16 tooth front sprocket
Needle Shim Mod to the carb
2 month old spark plug
89-91 octane gas (canadian ratings)
Air filter isn't WHITE but it is fairly clean ..little bit of oil on the part exposed to the carbs outlet port

I'm noticing lately after i warm up the bike and get going the sound of the engine sounds more ""hollow". not popping but ...something is off
the bike rides fine overall.. especially after the first 3-5 minutes but if i park the bike for awhile (10-30 mins) restart the bike and go it has that sound again... a baffled hollowness while bringing me through 5-10-15 kph into 2nd gear


Piston rings?... I'm sure in the distant future a engine rebuild we be in order after all of these big cross provincial rides sometimes at 110 kph in 4th gear to cut through the wind.

sound about right
Rides through the rain