Hey, now that I think about it , it does sound like marbles in a can but very faint. Like the can has a towel around it.
Any suggestions?

 

Bump for Chefstan

 

If you were rolling on throttle 6th gear 74-80, yeah if it has a high pitch to it during that roll on.... I'd say pinging, as that's what mine will do ( new TW-222 cams to be installed to remedy but that's another story).
But then, if at steady speed and there is no dectected indication of pinging, then yeah, it is probably the sound of air intake through the A/C.