This is tentative but at least addresses your symptoms: Try spraying some carburetor cleaner, starting fluid, engine cleaner, etc. near the whistle while running. If the idle speeds up momentarily, you have a "sucking chest wound". Replacing or resealing (with RTV) the intake manifold gasket should do it. Obviously something is opening up as the engine warms. Doesn't sound like the head gasket but you can spray around that area also. I haven't had my Volvo for long so I went out and looked at my 8v (red block) engine. The intake manifold gasket looks simple and easy to replace or reseal (if not damaged). You might even see some bubbles in the affected area (after spraying). And I am responding based upon my agreement that it sounds like intake vacuum. Changing engine speed should alter the pitch of the sound if so.