A couple days ago my '83 245 started making a noise when the throttle was opened from idle or to accelerate. The noise was an air sucking sound under the dash. The same noise occured when the engine was shut off. After investigating, I found the problem was the check valve in the vacuum line on top of the manifold. This check valve (black and white plastic disc with 1/4 inch hose barbs)prevents the vacuum chamber under the dash from pulling air out of the manifold when ever the manifold pressure drops (as it does when you accelerate. Throttle response also seemed to improve slightly after making the repair.

If you hear the sucking sound when accelerating and at shut off, this is likely the problem.