I removed the MAF and cleaned the upper seal between the MAF and the elbow and the lower seal between the elbow and throttle body. They each looked to be in good shape. When I put it back together and retested, I got the same hissing sound. I should have been clear that the sound is coming from that area but I can't be sure where it's coming from. It could be coming from the MAF vent hose... or somewhere else. I could just start replacing seals and hoses until the problem is resolved.