Hi i just bought my 1999 civic si and noticed a loud squeal/whistle under medium to hard accleration. My first thought was one of the belts is bad, but after closer inspection I found that the valve that opens up to let air from the intake in makes a slight squealing sound when I give it gas (not moving). Im not 100% what it is but any help would be great thanks.

 

either:
a) accessory belt
b) aftermarket intake squeal

check your belts for any type of wear or looseness and check to see if your intake is tightly connected and sealed.

 

b) aftermarket intake squeal
+1

 

Thanks for the response. It is a cheap ebay intake but like I said I think its that valve, because I took the intake off and noticed that that the noise was still coming when the intake was off. Ill try a couple things tomorrow. thanks

 

Well, the thing is, the factory intake is designed to "muffle" the noise that is emitted from the intake portion of the motor. When you add in a aftermarket intake with a short tube, you'll hear a slight increase in dB's when the motor is at idle from the engine bay. And as a note, you're referring to your butterfly located in the throttle body.