Sometimes the dryer will work when we hit the start button

ALWAYS UNPLUG YOUR DRYER BEFORE STARTING ANY REPAIRS
Your problem sounds like drum rollers have flat spots on them or in need of lubrication, or maybe your drum belt has slipped off of the idler arm pulley.
If you have a front loading machine,

Remove the electrical console that holds the timer switch, start switch, etc. from the top of the dryer.
Be REAL CAREFUL not to knock any of the wires off of the timer switch. Then remove the top of the dryer.
Remove the front of the dryer. This will expose the drum and belt. Remove the drum and belt and this will expose the drum rollers, idler arm pulley and motor.
Now is a good time to check the belt to see if it has slipped off of the idler arm pulley and also to check the belt condition for wear.
Be sure to note the route of the belt, this will come in handy when you go to reinstall the belt.
Check your drum rollers for flat spots or wear, if they have flat spots or wear on them replace them.
If the drum rollers don't have flat spots or wear on them then they probably only need lubrication. Lubricate the shaft of the rollers with a good grade of machine oil such as 3 in One oil. Reassemble the dryer, plug back in. This should solve your noise problem.
I will include these two websites for parts and further help:
http://www.partselect.com/
http://www.appliancepartspros.com/index.aspx
Both also have chat assistance 24/7.
Good Luck,
I hope this helps
Please rate me,
Thank You