Marg, do the blade chucks, clamps move up and down, eg closer together and wider apart, or another way, do you have to hold them closed when putting in a blade.
If this is the case, in the top arm is a mechanism that moves the top blade chuck up and down, on my Ex21 this is joined in the middle for some amazing reason. Quite a while ago, the nut that joins these came loose and I had a knocking noise. The nut that uses the saw is always loose