You wrote "sounds" like a dull vibration. Vibration doesn't make sound. Insufficient info to make any sort of guess. Mick is correct about old balancers. This could also be the sign of a valve head getting ready to pop off the stem.
George
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.