I just bought a used 93 Acura Integra LS special 2D with 119000 miles on it. I am newbie, and need help for several questions:

1: When I turn off the car. There is a strange sound from the back like a train Wo-W0-W0-W0-W0. It lasts three seconds. But if I turn the radio off, this won't happen. And later I found out that even the car is started, when I turn on/off the radio, this sound comes again. Anyone know what's the reason?

2. What's the use of S3/S4 shift? When to use it? The seller don't have the owner's manual, so I know nothing about it. Where can I find the manual? (It's great if I can download it for free. Poor student )

3. When the car pass bumps, it feels so hard, sound is loud... is that normal?

4. What kind of gas I should use for this baby? Need to add anything to it?

5. When I take the car to Wal-mart for oil change. They recommend to change the air filter. Should I buy one online or just ask them to change? (Which one is cheaper? And about how much for it?)

Thanks very much!!
Panda