Just adding a spring wouldn't help much, because the problem is that the swivel arm is too short for a block of that size. A quick fix would be to change to a plastic fairlead on top of the cleat:
At least it would reduce the noise... but the real solution would be to change to a larger swivel such as the Harken 205:
...or to some completely different cleating arrangement.
With these ideas in mind, I took another look at my Harken arrangement. That Harken may be too big: if my other Harken is smaller, I'll give that a try. (It's not in Florida presently).
Raising the Harken block puts the mainsheet at an awkward angle. (It would be too ready to release, or not grab at all—actually the reason the Harken is presently installed "bottomed-out").
A larger swivel would work, but also as suggested, a different fairlead will be my next try. The existing fairlead is generously tall, and I should have a few shorter ones to select from.
Thanks for the different "angles" to this problem.
American sail has a really nice setup that mounts like a rudder inside the cockpit. I haven't been able to find them aftermarket though. If you near them in Florida, it might be worth a trip to see if they will sell us a few separately from their knock-off boats.
Eddie, that is a standard Harken fitting - and it absolutely doesn't work in a mainsheet system on any boat, unless there is a lead above it (or it's attached to the boom). It can't handle the different sheet angles.