Framebuilders - Stuff inside frame

Hi... My neighboard came to me this morning asking me how to take a noise he has inside of the the tubes. I doubt it is flux because the bike it is pretty old... a trek something steel.. MTB something type of bike.

Usually heating a little bit the tubes will make the flux to melt and problem fixed but looks like this is something else. Wonder if it is sand or something that got trapped in the rear end.

Some ideas?

Thanks.

Assuming that all the tubes were sealed by seat posts, headset, etc, the first thing that comes to mind is rust. If the bike has spent a lot of time outside or was ridden in places where it can collect water, it's a possibility.
My second guess would be possibly a water bottle frame nut. Check to see if any are missing.

Regards
Mike

THanks... Nope nothing missing... It is weird looks like the stuff it is in the tube that goes from the seat tube clamp into the chainstays. There are a couple of little holes but nothing comes out, Thats why i was wondering if there is a trick to be able to take junk out?