The unstable feeling of the car could be due to the tires. I had problems when I upped the size to 245/55/16's all the way around, especially when they were brand new. The tires that gave me problems were Pirelli P7000 SS's. As the tires wore down, the problem went away. I don't know if this is your problem, but it could be.
As far as the sucking noise, that's just your engine breathing. It becomes quite audible once the air silencer is removed, especially with an open element. In fact, that's what the purpose of the air silencer was...to quiet the sound of the engine pulling air. Don't worry about it. When you get around to upgrading your motor, intake, and exhaust paths, it's going to get louder
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--nathan
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'91 GT, Coast 347, 9.5:1 compression, full intake, Wolverine 1087 cam, exhaust, Keith Craft ported Windsor Jr. Irons (235 cfm intake, 195 cfm exhaust), AOD, PI 3500 converter, Lentech valve body, 3.73's (4.10's in the works), and Yokohama ES100's out back.
Daily Car: '04 Infiniti G35 Sedan 6MT