Noise from heater hose

A friend has a '99 truck with a 350ci. A noise that sounds like a cavitation noise is coming from one of the heater hoses. This particular hose terminates into what may be some sort of valve into the
intake manifold. This one hose fits into what looks to be a large nut and that piece screws into the manifold. What might this be? There are no wires or hoses involved with this nut-looking thing. The other three hose-ends from the heater terminate normally. Any help? Thanks!
Dave S(Texas)
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