My 411G was hitting the presser foot. I tried everything finally, I replaced the bent needle shaft on my Singer 411G. It was not hard to do - it took about an hour - most of the time was spent timing the needle and the shuttle. I did not think the needle shaft was bent until I got it out and compared it to the donor's needle shaft. There is a picture on the bottom of this page:
http://www.quiltingboard.com/t-130994-6.htm and link to my discussion of it
http://www.quiltingboard.com/t-43881-1202.htm#3520792 and more on
http://www.quiltingboard.com/t-43881-1169.htm