[Inziladun]

Originally Posted by Lalwendë

I still have this niggling feeling that Aragorn knew what was in there while Gandalf did not, given his words to him before they decide to go through Moria.

I don't think so; Aragorn's warning seems to me to be based upon a combination of his own previous experience passing through Moria and a sense of precognitive foreboding about Gandalf's fate. When the Company is debating Aragorn says:

'I too once passed the Dimrill Gate,' said Aragorn quietly; 'but though I also came out again, the memory is very evil. I do not wish to enter Moria a second time.'

I believe Aragorn sensed the Balrog without knowing what it was. If he had an idea of its true nature, surely he would have mentioned it to Gandalf privately, especially considering that they had already discussed the possibility of taking the Moria route.

That may beg the question of why Gandalf did not sense the Balrog on his prior journey there. My theory might be that it felt the presence of Gandalf first, and did not desire a confrontation then. Perhaps it was the power of the Ring that called to the Balrog (and maybe too the Watcher in the water) when the Fellowship passed through, whereas Aragorn and Gandalf were unmolested when they went in alone.

Originally Posted by Lalwendë

As to why the White Council did not deal with the Balrog, it's a moot point as to whether they knew it was there. There was something that had scared the Dwarves away, but who would be willing to find out? And even if they knew, it was likely that they chose to leave it well alone - it had frightened the Dwarves out of one of their strongholds and what is that in comparison to the threat that Sauron had faced and would face to all of Middle-earth?

Indeed. The primary focus of the Istari was Sauron, and those who directly aided him. Whatever the evil in Moria was, it wasn't likely to be much concern if it wasn't seen as a ally of Sauron's, actively or potentially. After all, in UT Gandalf tells the hobbits in Minas Tirith that he was only minding the Smaug affair because he feared Sauron might use Smaug in his war effort.

Originally Posted by Rhod the Red

If I recall precisely, we have to specification of the reason Gandalf first visited Moria. So maybe he was sent by Saruman, or the White Council as a whole, to investigate that very matter.

Saruman's clearly someone whom likes the comfort of Rivendell, Minas Tirith and then Orthanc and Radagast is going native west of Mirkwood so Gandalf is the logical choice to go investigate.

No, Gandalf tells the Fellowship he entered Moria to look for Thorin's father Thráin.