My Amana model sx25sw (purchased in 1997) , side-by-side refrigerator is making a noise and I am not sure if it is the motor or the compressor. I cleaned the dust from underneath, the back and the front but it is still making the noise. Is there a way to diagnose this (the sound is similar to a helicopter)?

The condenser fan is probably hitting something. Remove the back to see what it's hitting, if that is where the noise is coming from. Here are some manuals: https://dl.dropboxusercontent.com/u/8010163/Maytag%20Refrigerators.zip
Your model is the side-by-side one at the bottom of the list. There are diagrams to help if you need help. Maytag makes your model, so the manual covers both names.

Thank you for your time. The product manual really helped. Turned out to be the condenser fan motor.

Make sure you keep the fan and the coils clean by washing them off with spray soap and water. This will cut your power bill and prolong the life of your refrigerator. The main reason (and not covered under any warranty) for total failure of a refrigerator is dirty coils. Refrigeration works on the principle that latent (heat of evaporation needed to be absorbed for a change in state) and sensible heat (heat you can measure with a thermometer) and rejecting it through the condenser into the air. Dirty coils stop this and cause the compressor to burn up.