Loki didn't just walk into any hall that wasn't his own without knocking. It was the polite thing to do. Not even his mother's home was entered without permission of some sort. It wasn't his home and he wouldn't be forward enough to just walk in. In fact, there were very few halls he would ever just enter without being announced in some way. And those all belonged to Odin in some way.
But he would never dream of just walking in Idun. For starters, it was rude -what if she was in the middle of something and he startled her causing her to break something of emotional value? Or worse, what if she was otherwise engaged with someone... she wasn't really that sort of goddess, but she was like a sister to him. The idea of finding her otherwise engaged with a man because he hadn't the common courtesy to knock made him uncomfortable.
And maybe just a little sick.
So he knocked, and waited patiently for her to answer. He had just gotten back from Muspell and was rather anxious to tell her how his daughters had grown. They were simply stunning to look upon and bright! They were such bright girls. Seeing Glut had sort of been a hardship he'd wanted to avoid -their divorce had not been a mutual decision but in the long run it had been for the best, but he would have done anything for Eisa and Einmyria. If that meant tolerating their mother for a time, so be it.
He'd also bumped into his father, albeit briefly, and left a sour lump in his stomach. Loki and Farbauti didn't get on, at least not from Loki's perspective. He had no tolerance for anyone who took advantage of his mother. Or any woman, really. Which is what his father had done.... twice. Both on Laufey and Loki was fiercely protective of his frail, weak mother.
All this and more he very much wanted to share with his dear friend, if she'd only answer the door.