My berinina sewing machine is making a clunking noise but the stitches look fine . how do i find out whats causing the noise?

needle /hook timing , incorrect needle thread tension or worn bobbin shaft

the best advice is having the machine serviced, but here are some other suggestions

Bernina 880 - the dual feed will suddenly start making a loud "clunking" noise and making very tiny stitches. Then just as suddenly it will begin to work again - but the stitching is clumped, uneven with loops on the back. Then it will start working fine again until the next time... I called the dealer and talked directly with the repair person. He had never heard of that before. Anyone have any answers? Did I get a lemon?

There are two things that come to mind that may be causing the problem.
1) Change the needle, for it could be bent even thought it may not look it. (This is the most likely cause).
2) Have a look just inside and check if any screws have come loose. This has happened to me. (Even the repair man did not find it.

have you checked to see if the bobbin is in the bobbin case correctly
is it still under warrenty? you might want to take back to the place you bought it from & let them check it

try changing the needle
check the top thread tension
check to make sure the bobbin is inserted in the bobbin case correctly
check to make sure the bobbin case is correctly inserted into machine

Check the bobbin to be sure it is inserted correctly. When you pull on the bobbin thread the bobbin should turn to the left(orcounterclockwise). If it doesn't, the thread will come out of the bobbin tension and give you loose stitches on top of the fabric. It will also make a noise because the bobbin is bouncing up and down.you will need to readjust your tension again.
sewman7

The top of the machine is not threaded correctly. First raise the presserfot then rethread the machine. Bottom looping is usually caused by a lack of top tension. There could allso be a bur on the hook.