There is some brown stuff inside the radiator.

It is embedded on the top and bottom of the radiator tank.

Any idea what this is?

And is it dangerous?

More than likely its rust if you are using water in your radiator, if you are get a few bottles of radiator flush and flush out your cooling system (follow directions on bottle) a couple of times then refill with a good quality coolant

hi man this a very common problem in most of the ford radiator systems dont know why but its just a little rust build up, which can be fixed wth radiator flush as EASPAK91 said. wat u do is drop all the coolant out of the radiator take the overflow tank off and hose the build up out then put the radiator overflow tank back on fill the radiator with water and radiator flush turn the car on and run for about 1-2 min or until the water has circulated the engine a few times then drop all the water out and fill with new coolant. but eventually the rusty build up will come back over time but as long as you have good coolant and not much build up it wont harm the engine... hope this helps good luck

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wats green and smells like yellow paint.. green paint

Thanks.

What is a good coolant to use for a Falcon?

I dont mind spending the dollars for a top of the range coolant.

Since I'm in a particularily helpful mood tonight ( I was going to say the brown stuff was s**t and the solution was to stop the baby s**t in your radiator, but thats not very helpful is it ?).
The best way to clean the tank out if it is fairly dirty is to:
A) remove whole tank and block off the various holes however possible; generally a kinked bit of hose does the trick. Chuck a handfull of small rocks into the tank and about 2 litres of water then shake like hell. repeat until clean. The hard part is getting all the rocks back out.
B) remove whole tank and block off the various holes however possible; generally a kinked bit of hose does the trick. If you can get some pool acid chuck a small amount in and leave sit for an hour or so. Repeat till clean.

I've tried both methods and both work pretty well.

WTF are you serious????????

ive cleaned them basically same as above, remove tank, a few handfulls of a*** sand,boiling water and shake and shake, flush out and repeat.
have also used the household cleaner CLR and some sand with boiling water

I have been seriouser (WTF: is that even a word ) but this method does work.