Excessive engine noise

Depends on what the noise sounds like. A whistle could be a cracked inlet tube around the air filter. Bad bearings in an accessory like the alternator, power steering.
Could be internal engine damage or transmission related. Brakes or tires can make more noise as well.

To eliminate items:
Does the car have to be moving for the noise to happen or can you hear it setting still with the transmission in park?
Does the noise get louder if you hold the brake on and put the transmission in gear?
Does it matter if the gear is forward or reverse?
Can you describe the sound better?

If the noise only occurs while moving:
If you lightly step on the brake, does the noise change/stop?
Does the noise continue even once you get up to speed or only while you are speeding up?