Odd noise while driving

Hey all, 2000 Gt here. I recently started hearing a sound almost like slowly letting air out of a balloon and a slight whining sound with that. It's very hard to explain the sound, but I'll note that a while back I changed my air filter and I didn't get the cover back on all the way(b*tch on the v6). This caused a very similar sound while accelerating and coasting slow enough the hear the sound. When it happened with the filter housing, it sound more like an actual vaccum cleaner. Could this now be a vaccum leak? I don't really have any idling problems.....just poor mileage - like 14-15 mpg city. I don't drive hard either, I drive like an old grandpa now thanx to my license suspension/revocation. I just had my lower intake manifold gaskets fixed under warranty and had them change the fuel filter at the same time. Plugs and wires are fairly new as well. Any tips are appreciated, thanx.

Best i can suggest is take a good listen to engine with hood up, and try to pinpoint the sound. Most likely it a loose or cracked tube or hose, although a very slight possible that air is escaping a gasket.

Your guess is correct, you have a vacuum leak. You should look at all vacuum hoses. You have either developed leaky hose or one has come loose. This is why your mileage has dropped so far.

Quick trick to check for vacuum leakage, get a spray bottle filled with solution of dish-liquid and water, try to keep it unsudsy. Spray on suspected areas, the solution with either get sucked in or will begin to bubble as air escapes through the solution.

Keep spray away from any electronics, though.