...Could be a spun Rod bearing. When I spun a Bearing...it started out as a Tick, as in a Exhaust Leak or Lifter and advanced into a 'Rod-Knock'. Could be the #1 MAIN Bearing.
My Guesses: The sound seems to be Pulsating/Intermittent.
1. Water Pump Bearing. Perhaps we're hearing the Idler Pulley Bearing. Or the WP Impeller is Rubbing against the Plate, because the bearing has worn-out.
2. Distributor Shaft Bushing and/or the Dist. GEAR is Not getting adequate Lubrication and is Eating itself up against the Cam. Or the Wrong Dist. Drive gear is just now starting to fail.
3. Which brings us to Timing Chain and Sprockets. When these chains 'Slack', they can rub against the water-pump PLATE. Especially if they are the factory 'Nylon covered Teeth' type.
4. Fuel Pump 'Eccentric' has no lubrication against the Pump Arm. Or the Fuel Pump Itself.
5. The 'Oil-Slinger' is Rubbing against the 'Cover', if installed backwards and has Now, slipped into contact.
BEFORE You start tearing things apart, First, get a Mechanics' 'Stethoscope' and try your best to Pin-Point the Cause of the Noise...Most Caution for the Alt Belt.
MJ