Snapping Noise from Heater Core 85 Caprice

Hi,
I've got a crackling / snapping sound coming from my heater core. If I pinch off one of the heater hoses, the noise stops. The noise comes
on acceleration, and at a slight rev in neutral. Causes I can think of :
1) Steam is forming somehow in the cooling system, and I am hearing the old-fashioned steam radiator sound in the heater core. 2) Air bubbles are trapped in the heater core and are slapping around in there. 3) A piece of debris (metal) is flopping around in the htr core.
Additional info : I do have a coolant leak at the rear of the intake manifold. Maybe air is getting into the system there.
The car is an 85 Chevy Caprice. The heater core is only 3 years old.
Any ideas ?
L
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That water leak scares me. My V8 experience tells me a leak, either front or rear, allows water to enter lubrication system. This will sludge up engine, and ruin bearings. The noise sounds like steam.
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How would steam be forming in the heater core ? My coolant is 50/50 coolant and water, and the car is not overheating.
Thanks
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