Installed my new subs tonight and I think I have a problem. One of the subs, when turned up loud, makes a squeak type noise every time it hits. At lower volumes everything sounds fine. Anyone have any idea what is wrong?

 

It sound like the crossover settings aren't right. I have seen this happen when the settings are off. You need to readjust the Gains and the crossover fequency on the amp if it has them.
Basically from what I understand about these things, your subs cone is being over excurted (extended) which could cause some serious damage to the sub. I would suggest bringing the freq level down. Depending on what you are running, you should be under 180Hz.

 

More than likely the sub needs some "break in time" Just like a new engine. Cranking it up as soon as it goes in can cause uneven wear just like a new truck. The voice coil needs to wear to the gap. Give it a little time and make sure that the gains aren't to high.