My initial guess would be a vacuum leak.

Pull the engine cover off and start inspecting. Use a spay bottle with some distilled water (so it doesn't leave mineral deposits) and lightly spray a stream (not the spray pattern) of water (with the engine running) around the mating surfaces of the intake manifold, hose connectors etc. If you spray something and the noise stops for a bit you found your leak source.

Tighten up any loose bolts or clamps and if that doesn't help you might need a new gasket (if one is used) or further inspection might reveal you need something else.