I've recently heard of a timing chain tensioner going bad and having chain slap occur. I don't know what it sounds like but if it comes from the front of the engine then that may help.
Former owner of '99 CSVT - Silver #222/2760
356/334 wHP/TQ at 10psi on pump gas!
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'05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red
'06 Mazda5 Touring, 5spd,MTX, Black