The Nissan Path

I have an 06 SE that had the radiator and transmission replaced a few years ago with 75-80k miles. I now have just over 100k.

For anyone who has also had the transmission and radiator replaced due to the coolant leak, has anyone noticed residual contamination in the system after the replacement?

I've tried to flush it out of the radiator and got a ton of gunk out but there still seems to be some that accumulates under the radiator cap.

I had both of mine replaced this year, and the dealership told me they had to a full flush, test drive and repeat four times before it came out clean. Every time they also flushed out the coolant out of the engine, cause that was contaminated every time too.

Only put a few thousand K on it, but it looks sparkly clean still.

It all depends on how quickly cross contamination was discovered. Antifreeze in tranny causes a lot of damage, it eats friction material from clutches and rings, it eats gaskets and finally it builds up in corners with lower fluid flow.

If contamination was discovered via transmission malfunction, I think that's too late for just bypass and flushing, as vital parts of tranny are already damaged.
But if it was discovered by regular weekly coolant check on radiator cap, then simple fix and flush might be sufficient.

Since you have had radiator and tranny replaced, the gunk you see is most probably really some residue, peeling off and washed out by fresh ATF and fresh coolant. As SETI says, repeat both ATF and coolant flush after few thousand miles again and see what comes out.