To perhaps better locate the source, try a mechanic's stethoscope or a listening stick - piece of wooden dowel, touch it to various spots on engine until you locate and get closest to the noise source. Old broomstick works.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.