The air bypass valve usually just sticks in some odd position (sometimes open, sometimes closed). If you wanted to eliminate it as a suspect, just plug one of the hoses - it pulls filtered air in from in front of the threottle valve, and releases it behind the valve.

It sounds like an air leak somewhere in the intake system to me. I'd double check that the throttle valve is able to close completely - when they get worn or if the valve disc gets loose on the shaft it will tend to hang open slightly.

Other than that there are the various fittings on the manifold - for example the brake booster - it could be leaking and sucking in air. The manifold gasket on the head. The injector base seals.
--
I'm JohnMc, and I approved this message.