It sounds like when you are viewing your "raw image" before opening it in Adobe Camera Raw you are actually looking at the JPEG preview that is generated in-camera and attached to the raw file. Whatever settings are set in camera at the time the image is captured are applied to the raw data to produce the preview image. Along with color temperature/white balance, contrast, saturation, etc. this normally includes noise reduction as well.
The reason this is done is because you can't really view the raw data itself as an image the way we expect an image to appear. Raw files contain brightness values for each sensel (pixel well) on the sensor. The relative brightness value of the signal from each sensel is affected by whether that sensel has a blue, green, or red filter in front of it. Even if we did try to look at the monochromatic luminance values from each sensel, our camera's lcd screens and computer screens aren't capable of displaying 12-bit to 14-bit gradations in brightness. The signal from the sensor also needs to be converted from linear response values to more logarithmic values on a light curve shaped to match our eyes' response to various brightness levels.
The jpeg preview is what you see when you look at a "raw image" on the LCD screen on the back of your camera. Depending on the settings of whatever application you use to view a raw file the jpeg preview can also be what you are seeing when you look at it on your computer, too.
When you open the actual raw data in Adobe Camera Raw the default settings in ACR are used to interpret the image data. This may have more or less noise reduction applied than was used to create the jpeg preview. It sounds like in your case there is less. If you want to apply more noise reduction then you'll have to use the controls in ACR to increase the amount of noise reduction. There are two basic type of noise: luminance and chrominance. Luminance noise is what makes images look grainy. Chrominance noise is what make colors look 'speckly.'
Changing other settings can also increase the amount of noise you see. Increasing saturation also increases chrominance noise. Boosting exposure/brightness increases noise because the noise is amplified along with the signal. Sharpening an image will make noise in the image more noticeable.