First coolant change - $#%+ In coolant??

[Why Z?]

Hi all,

Did my first (since bike has been mine) coolant change this Sunday.
Color came out a kind of dark grayish green.... (WTF!!?)

As it settled you could see what almost looked like little grains of BLACK SAND in the bottom of bucket (they did NOT FLOAT) and then the coolant in top of it resembled a more normal color.

Any ideas?
Bike is an 07 EC450 FSR. It was a research bike and then rebuilt. When I bought it it had been "fully rebuilt" (from one if the guys from the company was testing it). Note, he had the radiator repaired due to the bike getting slammed in the crate.

What do you think this stuff is?
Is it serious? (Right now plan is to flush it w vinegar/DW n then ride)
I have googled the p*ss out of this and only thing I can find is people with gasket/hose issues.

I don't think it is rubber/gasket/hose. (Doesn't float)
I don't think it is iron/steel corrosion (Not brown, doesn't seem obviously magnetic)
Also, I checked the impeller and it is sold in there so I don't think the impeller or shaft or bearings are jacked.

So I tried to figure out but now I turn to you guys

[TheLetterJ]

Casting sand from the factory that had not been washed out well enough?

[gasgasman]

Probably gasket material that was scraped off during the rebuild.
When using an air grinder (with an abrasive wheel) to remove the gaskets, the fragments would look like sand.

[Why Z?]

So... Something to worry about or no?
Anything I need to check/replace?

Or will a coolant flush be sufficient?

[gassergrant]

Originally Posted by Why Z?

So... Something to worry about or no?
Anything I need to check/replace?

Or will a coolant flush be sufficient?

Flush out the system with a garden hose and see what comes out, my thoughts was old gasket to or if the rads have been welded could be that
Flush clean and refill use and just keep a eye on it

[bergerhag]

Could it be residual blasting sand from a cleanup blasting?