It'll be the Nut Loosening Pixies.....
Little buggers they are! I had a Fraud Crappi 3.0L years ago (Yup, Sad, I know....) and the starter bolts would loosen off over a week or so, allowing the starter to shift and make awful noises when starting.
Tried all sorts. Even bought proper new Fraud bolts and lock-washers for it. They lasted a month before they too fell out...
Never ever did cure it....
Sold the thing for more than I bought it and bought a 2.0L DOHC Toyota Celica RA40--which was faster and more economical than the Fraud--Loved that car!