Predictable, in the sense that I *should* have been able to predict it, but didn't.
So, as part of a vacuum system setup I need an outlet exhaust pipe from the rotary pumps, to outside. About 6 m of tubing, not much flow rate, far end vents to atmosphere so there's virtually no pressure differential. Any old tubing will do, so long as it's large enough diameter to present no significant resistance, rigid enough to not kink flat on bends, and yet still flexible (since the gear can be moved around.) I'd settled on 18mm garden hose fittings for the quick-connects.
Oh, and CHEAP. Of course. Yesterday at Bunnings the 18mm garden hose is far too rigid, and also nearly $40 for 10m. Turns out corrugated '22mm gray water' hose also fits the connectors, is very flexible, and only $10 for 10m. Yay! Just needs a little help with sealant at the joints.
Luckily, when I got it home I happened to try blowing through it. Guess what?
Turns out a long length of corrugated-wall tube makes a GREAT whistle. Weird sound and kind of cute, but I don't think I want the vacuum system to make this noise during pump-down.
A short length of the tube doesn't whistle, so I suppose it's a turbulence resonance amplification thing, that builds up along the tube.
Anyway, back to the tube hunting.