Noise from bobbin case on new Janome 4020

Question

I have just bought a Janome 4020. I tried the different stitches, when I used the overcast foodt it started making loops on the top-side of the material and it started to clunk. I adjusted the tension. Then the thread wouldn't come freely .So... I rethreaded it, took out the spool and cleaned the casing. It doesn't clunk exactly but is quite noisy, just like an old Singer. Can you tell me what it might be??Many thanks