Today the driver side front power window in my '87 300E stopped working -- when I push the down buttom, the window goes down about 1/4 inch and then it stops and makes a loud, repeated ratcheting sound but won't budge. I can make it go back up to the top but it will do the same thing when I try to make it go back down. Any ideas on what this is? I will try to fix it this weekend. Thanks for any help.