Waterfall sound under dash

[91Sportside]

My 2001 Silverado, 4.3L, manual transmission has a waterfall sound under the dash when driving. I suspect air in the cooling system, but don't know how to get it out. When I take the expansion tank cap off, there is no pressure, the truck doesn't overheat and the heater blows good and hot.
I suspect that I also have an intake manifold leak, but have put a bottle of BG cooling system sealer in the system and the leak seems to have stopped.
Any ideas?
Steve 91Sportside