Off the top of my head I would think that if the Immovable Rod was being used to bar a door then the Knock spell would indeed work on it. The fact that it is it's own separate item doesn't (IMO) remove it from the category of "A target that is stuck or barred becomes unstuck or unbarred."
And the spell description block may even support this, "If you choose a target that is held shut with arcane lock, that spell is suppressed for 10 minutes."
This sets the precedent for the Knock spell temporarily nullifying a target that is locked, barred or stuck magically. If it can suppress another spell, why not a magical item?
I'm of the mind it's a no, and it's due to the power of the magic in question. I do, however, agree that other magical means of barring a door may be thwarted by Knock.
Arcane lock is a 2nd level spell, and Knock will only suppress the spell, not remove it. Immovable Rod is a magical item which is significantly more powerful than a 2nd level spell. I would argue that because of that gap in power the Immovable Rod would shrug off something as simple as a Knock spell.
Simple question but I can't find anything decisive on it," Does Knock work on an immovable rod?"
I would say no as well; the immovable rod in itself doesn't "prevent access". And if that were the case, it is a definite seperate object from the door (unlike a bar that's been attached to said door), so targeting it would be impossible from the other side of the door. As VillainTheory said, antimagic field would definitely work