I have a Maytag dryer. It makes some knocking noise when it starts. What could be the problem?

You have to do a little research here.

To check what is going on in your dryer :

Remove the rear panel, and have a look as to what is going on.
Check the drive belt : is it frayed?
Is the drum running against something?
Does anything look lose or broken?

Please be careful : do not touch anything while the dryer is connected to the power.

If you can not find anything, remove the power - take the power cable out of the socket and put it on to the ground.
Note down how the belt is connecting the drum and motor.
Remove the belt (by taking away the tension from the belt this is easy).
Spin the (empty) drum a couple of times by hand, as fast as possible. Is it running freely?
Spin the motor a couple of times by hand, as fast as possible. Is it running freely?

This procedure will allow you to research what is the problem in your machine.
If everything seems fine your problem may be related to either the drum bearing or the teflon "gliders" between the drum and the door window.

Of course you can also come back to the board and discuss your findings!

Success !

:)