Thank you for a very good question. Depending on the make & model will depend on where to start checking for the loud squeaking noise you hear. On most dryers I recommend checking the drum supports & bearings. If you have a GE dryer the first responder's link is a good place to go for that model of dryer.
Be sure to
before attempting access.
If you have a standard Whirlpool built Kenmore dryer I am sending you some helpful troubleshooting tips showing how to access inside the dryer. On this model I recommend checking the drum supports (rollers) and idler pulley.
Please reply back with the model number so we can try & help you solve that squeaky dryer problem.