How would I go about freezing the rattles in place so the lure becomes silent?

you could drill a hole in there and then squirt some superglue on the rattles to keep them in place and then seal the hole with epoxy.

you might be able to drill a large enough hole to remove all the rattles and then re-seal it, but then you risk the bait not running properly because it does not have all the necessary weight.

To silence a bait seems simple enough but keeping the balance is not. If you just put in a glue then you must be sure that the rattles are centered inside the bait while the glue sets or the balance will be upset. One way to quiet them down is to add a small amount of mineral oil inside the bait, does'nt totally silence them on all baits but it does quiet them down and does not upset the balance, since it will tend to self level. If the weights are just balls they can be removed, weighed and the same weight of epoxy used to replace them.

What type bait is it? I have a lot of baits that I have cut apart and if its one that I have I can get you a picture of the inside.

Certain baits are weighted in the nose so you can just drill a hole and then remove the BBs because they are just there to make noise. Other kinds the BBS are there to help balance the bait, on these it is  not smart to do anything because you can seriously hurt the action.

I believe that Rapala lipless cranks are weighted in the nose so I would work with those    

I've always been curious about trying this on a rat-l-trap.  Where abouts would I drill the hole? From the top? bottom? side?  What size drill bit or does it not matter?

This is a rattletrap but not all of them are quite the same. You can see where the holes can be drilled and where glue can be added but realize that if you can't get them centered your lure will be out of balance. The front 3 balls are lead and they do not move, the rear has 27 1/8" steel balls that are loose in the rear chamber. The only sure way to know exactly what your dealing with inside a bait is to look inside.

What's the best way to get those 1/8" balls centered?  I'm thinking, I stand it on it's belly and squirt some superglue into the body cavity from the top.  Would that work?

It would be tough to get them all centered. What I would probably do is drill a 1/4"  hole right behind the line tie, remove the balls and drop in 3 or 4 1/4" lead balls. The 1/4" balls would lay inline on the bottom of the bait and a couple drops of epoxy would hold them in place. You might also just fill the cavity part way with epoxy and get the same result, more than one way this could be done.

Seems like it would be easier to just buy a non-rattling lipless crank like the Sebile Flatt Shad.

http://www.***.com/descpageSEBLB-SFS.html

Take the bait and drill a hole behind the line tie or right in front of it. Then hold the bait at a 45 degree angle or so. Just picture the bait swimming and hold it at that angle. Then add a couple of drops of super glue. After that, fill the hole with epoxy and touch up with paint.

Seems like it would be easier to just buy a non-rattling lipless crank like the Sebile Flatt Shad.

http://www.***.com/descpageSEBLB-SFS.html

;D  point taken.  But what fun would that be.

Thanks for all the pointers.  I'll give it a shot and see what happens.

If you decide to remove the steel balls I can tell you that you can get a real good seal by filling the hole you created with a dowel that has been sharpened by a pencil sharpener, sand, and seal.  I know that I have done over some $1-2 walmart baits this way with great success.  I bet the idea of mineral oil and the existing weight would work great, be cheap, and easy.  Epoxy just seems like it would make a mess, throw off the balance, and make a mess.  I also think getting epoxy in a little hole might get messy.