Engine Elusive Engine Tick

2003 GT with over 100k (single owner: me)
No misfire
No trouble starting
Fuel mileage normal
No smoke
No oil loss
No metal or discoloration in oil.

My engine has slowly started to develop a tick. It only seems to happen outside of a certain temperature range. The tick increases with RPM and gets quieter under 1k RMP and almost disappears after the engine has warmed up (but not always). It can occur at any time of day but mostly in the morning.

I've been considering dropping a bottle of marvel mystery treatment in the engine oil.

Any thoughts or suggestion? I'm having difficulty finding time to take it in to a shop with family and work issues. Any solid advice is appreciated.