Sounds like it might be a stuck/collapsed hydraulic adjuster in one of the rocker arms to me.
I would suggest you get a mechanic's stethoscope, remove the engine cover and listen up and down on both valve covers.

Are you sure Mobil1 0/40w has always been used ? Cheap oil can cause this by building up carbon/gum deposits in the tiny little holes the lifter adjusters have.
I have an 89 Ford Probe ( Mazda MX6 ) that had a noise like this, I removed the rockers and manually cleaned out all the little passages, fixed 'er right up.