Classic & Vintage - My Seat Stays sound like Maracas! Help!

I just received a used steel Merckx frame through Ebay and I have notice there is rust in the frame. I have cleaned out the seat, down, top tube and chain stays but there is a rattle of debris coming from the seat stays. Unfortunately, there is no way to get into the seat stays (as far as I know.) And the rattle of the debris inside the stays is going to drive me nuts.
Any recommendations or magic tricks to fix this situation.
Thanks!

There's no communication between the seat stays and the stretcher where the brake is mounted? If not, that suggests to me that the stays are about to give up the ghost, 'cos the only thing that could be in there in that case is bits---quite a lot of them, if you're hearing them---of the stays themselves!

There's usually a couple small vent holes near the bottom of them. Get some frame saver and spray a bunch in there, more than usual, and hopefully the wax will stick the bits to the side so they don't rattle around.

I had a frame recently that had no rust anywhere but there were some bits of something rattling around in the seatstays. I doubt it was rust. It would have had to be seriously rusted in order to create chunks big enough to flake off and make noise, and there was no evidence of rust inside any of the other tubes. It could be something else, but in any case, framesaver it and that should help stick it and if it is rust should help stop it from getting worse. I personally wouldn't worry about it being on it's last leg unless the other rust you found was really serious.

I agree with TimJ here. All it would take is for a tiny bit of debris (teeny pebble, grain of rice, etc.) to get into one of the tiny evaporation holes, and you have the makings of a percussion instrument. I have a fork that has this - there's absolutely nothing wrong with it structurally, but if I were to put a couple more of whatever's in there, it would sound almost like one of those rain-sticks.

it is dried bee saliva. Ignore or get a boom-box and some wine to drown out the sound.

I am going to spray frame saver through the little vent holes and see if that does the trick. The frame wasn't all that rusty so I am hoping the seat stays aren't on their last legs. I'll ride it until it breaks either way!

Get some castanets and a guitar and form a rolling mariachi band.

I have a frame that from the day I bought it, from the factory no less, made noise as I rotated it, sand in the seat stays form paint prep, been able to shake out stuff over the years, might be that rather than rust flakes and or a bit of brass rod if it is really noisy.

Bought another famous frame that the original owner thought a bit of brass was loose in the top tube, owner wanted me to know about it just in case it might bug me too much. There was a seed in the top tube, I discussed it with the seller after I had removed it, he was surprised, and conjectured a "buddy" at the shop he worked at long ago was the culprit.