Are you getting the audio to the Amp from the Standard Speaker outputs from your head unit or are you using the low-level (non-amplified) outputs?
Amplifiers need clean (low-level) sound in order to work best. If you're providing audio input to your amp from the standard speaker cables that come out of your head unit, the amp will deliver poor sound quality & often engine noise.
If you're using the low level out's from your head unit, then try the following:
1. Make sure you've got clean power. Connect the Amp's power cable (through a fused link of some sort) directly to your car's battery. Don't attempt to tap in to the power from anywhere else on the vehicle (not only will you likely void your warranty if the car's new, but you'll also run the risk of screwing up your car and getting dirty power to your Amp). Garbage in, garbage out!
2. Make sure you've got a great ground (must be the same size cable as the power, must be securely connected at both ends, must be making a good connection to bare metal - I mean use a steel bristle brush or screwdriver to scrape off paint, clear coat, color, anything other then clean bare metal may not work).
3. Try relocating the audio cables away from power cables (both for the amp and other vehicle electronics). Example: BMW's have the battery in the trunk. The power runs down the right side of the vehicle, under the kick plate below the carpet kit. If you run your audio cable down the right side of a BMW, you're bound to get engine noise from the power cables. It's best to run the audio down the driver's side to avoid ambient noise bleed.
4. If you're sure that both 1 - 3 above are good to go, try getting a noise suppression kit - this company makes some of the best:
Pacific Accessory Corporation