1. There are no codes
2. All fluids are full
3. No misfires nothing
4. Cranking seems more strained than usual - a little noisier when cold but still does not seem to be the actual engine - the engine is not "strained" or "bogged down" once started engine also idles at proper RPM
5.. Seems to be coming from under the car as if it was something odd like a flywheel or catalytic converter
This started yesterday after a long road trip home - I didn’t hit anything that I know of and have no other guesses
6. This is a 1998 XK8
Edited to add links to videos - any help appreciated
Edited again: Links to video were not active

 

Well its very light and tinny so not the torque converter heavy grumble. I guess its a heat shield come loose or baffle in the exhaust though it sounds more like it's in the air than inside a silencer ....if you get under you should easily find it.

 

+1 to Pistnbroke

Probably a heat shield worked loose. Just follow the exhaust pipes. Also check all of the thin steel pipes running under the car from rear to the engine. The car is at an age that their plastic retainers are breaking and the pipe could be rattling against the body.